Hello there. Out of curiosity, I want to ask how do you test your own creations for the game: classes, races, monsters, magical items to see how strong or weak that are and tweak accordingly? Do you have a set of adventure that you solo in order to check prior running your game session or you just go with the flow and tweak on real time?

If it’s a one shot doesn’t matter too much. But for a campaign, I like testing prior and like to have a preselected one shot to tests my own creation.

How about you?

I tend to base new things on existing systems. That helps me keep it balanced and gives me something to research for strengths and weaknesses.

Then I put it in my campaign. My friends and I talk about it as it comes in, and then give feedback, tinker, and/or remove.

I also use existing baselines when I homebrew, just to make it easier. After enough years of DMing, and enough time with my players, I have a pretty good instinct for what will be balanced and what won't work. When I'm not confident, or when my homebrew is really out there, I send my prototypes to a friend who's been DMing for like three decades to get his feedback.

For the times I really goof up, I'll just fix it on the fly. Most of the time, I underestimated how strong my players are, and I need to do some behind-the-screen magic to make the challenge more appropriate. I don't think I've ever nerfed anything post-release. If I give my players something that makes them OP, it just means I get to hit them that much harder in combat. Win-win, really.

I got some real wild homebrew stuff in my world, I don't have the time to run an adventure with all the items, and even if, my players have their own perspectives and ideas I will never account for.

Little general tip: Be aware that you don't step on anyone's toes. If you have a rogue in the team don't give the fighter an item that gives them Cunning Action. Makes the Rogue feel less special.

Now, before I give a player an item with dubious balance I will tell them. If I nerf the item later it will not be to punish the player, but because I do not want the others to feel bad/underpowered. They understand it, especially after explaining it like this.

Special situation for me, and maybe for other DMs:
Everyone gets an Artifact. I did want the Artifacts to be especially great for a player. That means going down memory lane on things they complained about or wished for. I asked a player 3 years ago what spells he thinks are cool that he doesn't have access to. He doesn't remember that conversation, I did. One person complained about all spells he likes being concentration, boom, concentrate on 2 spells for some time.
For one player, well, he seemed pretty content with his character, so, I did just ask him, and he told me he likes when numbers get bigger, and he can roll more dice. Done my friend.

This is basically everyone getting an overpowered item, but it only seems overpowered from their perspective. That sometimes requires talking to the player. It's not a surprise anymore, but it's worth it for their long term happiness.
You don't even have to be too specific. I just asked one player if he enjoys his obvious role as a supporter, and he told me he actually enjoys the strategic combat the most, so I rewrote his Artifact from being about boosting his supporting abilities to giving him Arcane Archer options.