I play without alignment because either they decide on an alignment and later play completely different or they decide on one and then try to adhere to it on all costs.

But for some things in D&D 5e, I still need to know the alignment. (for some Spells, Afterlife, other planes, Items, Artifacts)

Are there some tricks, some kind of psychological cheatsheet, to quickly decide on the alignment?

I generally solve that by changing the alignment requirement to specific behavior requirements, or just ignoring the requirement entirely.

The players’ alignment is irrelevant, and IRL people don’t actually have alignments. You just need to know their characters’ alignments.

Consider their beliefs. Do they belive in the virtue or law in its purest ideal, or Chaos in its purest ideal, practical considerations aside. Do they believe in their duty to help those around them,  or will they do anything, hurt anyone, just to get ahead? That should help you figure out the axis.

I think of Lawful/Chaotic as instead being Predictable/Unpredictable. A Lawful character will do things a certain way - they might always offer help, or always extort the NPC's before they leave, and that makes them predictable - when you're making the encounters, you can usually anticipate how they will behave. A Chaotic character will behave in all sorts of ways - they might adopt the goblin, or they might kill it - you have no way of knowing before they act. That's the Unpredictable part. Neutral characters tend towards predictable, but can be swayed by pursuasion to do other things - they tend to be the ones who don't make the decisions, and are happy to go along with the party's concensus.

Good/Neutral/Evil is down to what their actions tend to do, and is far easier to work out. Good guys help others, evil guys help themselves, neutral guys will do a bit of both.

So yeah, You can easily then say whether a character is Predictable Good, or Unpredictable Evil. It's far less subjective than Lawful (which people often think means "obeys the law")!

For ease:

• Lawful = believes in order, tries to respect others belief in it, thinks chaos is bad for everyone. Thinks laws can solve problems.
• Neutral = hasn't thought about it much either way, probably seems lawful but only really follows the social order because it's convenient
• Chaotic = has no regard for rules of any kind, wants to live free, actively wants to break free of social constraints. Thinks laws cause problems.
• Good = Strong sympathies and empathy, believes it's best for everyone to be treated as they would be
• Neutral = Not important to them either way
• Evil = Strong self-serving qualities, little empathy, no aversion to cruelty

Most real people would be Neutral. They follow laws because they get punished if they don't. They don't really care much about anyone outside their immediate circle of acquaintances in any significant way.

Alternatively, consider people who do certain professions will fit certain categories:

Lawful Good: Human rights lawyer who works pro bono
Neutral Good: Charity owner
Chaotic Good: Greenpeace Activist
Lawful Neutral: Insurance company owner
Neutral: Call centre worker
Chaotic Neutral: Graffiti Artist
Lawful Evil: Venture capitalist who exploits other companies
Neutral Evil: Stockbroker
Chaotic Evil: Hitman

Another fun litmus test for the Law/Chaos axis Matt Coville talked about once was the question: "if your character was at a red light in the middle of the night, and nobody else was around, would you run it?"

One way of looking at it is this:

If your character acts primarily for the benefit of others, they're good. If they act to better themselves, they're evil (You hear that Chaotic Neutral Rogues and Bards?). If your character has a moral code that they abide by, they're Lawful. If they prefer to do whatever they feel like in any given moment, they're Chaotic.

Good and evil in this alignment method isn't based off of the moral codes of the universe (Celestials V.S. Infernals or anything similar), but rather, off of one's personal desires. Selfishness vs Selflessness.

Having an "evil" character in a party doesn't necessarily mean they're actively working against the party, it merely means they're working with the party for personal benefit.

Are you willing to suffer harm or make sacrifices to benefit others? If so, you’re good.
Are you willing to harm others for your own benefit? If so, you’re evil.
Otherwise, you’re neutral with respect to good and evil.

Do you think individuals should be expected to give up freedoms for the benefit of society as a whole? If so, you’re lawful.
Do you think you are better off retaining your individual freedoms by living outside of society? If so, you’re chaotic.
Otherwise, you’re neutral with respect to law and chaos.

AngryGM

A lot of the advice being offered seem to be more on the choosing an alignment based on what a character believes, rather what the player believes the character believes, which is all well and good before the game starts. 

However, I believe the OP DM is asking how to assess an alignment after the game's been played a bit (like even mentioning "afterlife" considerations).  So actually, in this case, there's the luxury of not having to have the abstract "what does this PC believe" sorting hat of bad analogies informed by worse "real life" stereotypes.  Instead, assigning an alignment in this situation has the luxury of being able to be discussed and assigned by holding the PC accountable for their actions.  Alignment isn't just, actually isn't at all rationalization.  The descriptors are there to classify the character's actions.  Remember in the default D&D cosmology good and evil and law and chaos are akin to elemental forces, and alignment mapped this way allows the player to realize what forces their conduct really serves.  Law and Chaos, overall and in key campaign moments, has the character acted in accordance/reverence toward a code or system or were they more disruptive or disregarding of systems, or were they sorta in a the "meh" grey area?  Good and Evil: were the characters actions altruistic/benevolent/heroic or selfish/malicious/villainous?  Accountability sorta trumps player agency in this regard, and if the players don't land the way they thought they did, they've learned a bit more about the morality of your world.  IRL we call that a reckoning, and it can be a hecka fun thing to have a in TTRPG.  Maybe they'll radically change to be their "best selves" or maybe they'll embrace the revelation the mirror grants them.

Take notes on the party.  If you want to get all moral calculus even note where their conduct lands on the two axis every session and key choice moments and weigh accordingly.  The DM's an adjudicator after all.  Own it.