Gut feeling, eyeball, best guesstimate, and close-enoughing.
The DMG has a few pointers, but mostly it comes down to comparing it to the nearest rough equivalent spells in the existing game and deciding where your spell sits relative to them, and what sort of resources it should cost to cast. If you provide the spell text for the spell you're working on, you'll collect several opinions on where it should fit level-wise.
Make the spell then compare to other spells. Generally it is not that hard. Keep in mind that the more stuff you add on the higher a spells.
All of the following will likely bump it a level:
Damage types beyond: acid, cold, fire, lightning, or thunder damage
1/2 damage on a save rather than negates (because guaranteed damage)
Not needing concentration for an ongoing effect.
Doing damage AND another debiliting effect (Restrained, incapacitated, etc.)
Longer range
Area effect larger than similar spells of the same level
Being able to select multiple people instead of just an area effect.
Any spell that 'synergizes' with a class ability should be weaker than a normal spell (i.e. a divination wizard attack spell - Expert Divination, a druid spell that works well with wildshape, a warlock spell that does wondrous things in darkness - devil sight, a sorcerer spell that increases in damage the farther the victim is from you - distant spell )
How should I determine the spell level for a custom spell?
Gut feeling, eyeball, best guesstimate, and close-enoughing.
The DMG has a few pointers, but mostly it comes down to comparing it to the nearest rough equivalent spells in the existing game and deciding where your spell sits relative to them, and what sort of resources it should cost to cast. If you provide the spell text for the spell you're working on, you'll collect several opinions on where it should fit level-wise.
Please do not contact or message me.
Make the spell then compare to other spells. Generally it is not that hard. Keep in mind that the more stuff you add on the higher a spells.
All of the following will likely bump it a level: