If you're making the team sounds like you already have the right idea. Stuff like Solo Flawless and harder raids are a good indicator. Then you can run something with them or have them stream to get a good indicator since you're not looking for someone who needs to rely on cheese for every step. You still have to sort out who only plans to be there for a few hours vs the full 24 if needed.

Even if you're just joining an existing group you'd want to make sure it isn't a group that isn't what they claim to be. D2 is famous for the worse players recruiting great players so they can get carried. The serious ones are gonna have a stockpile a mile high of the best rolled weapons for every possible situation in every slot and several potential builds to roll with. They also shouldn't shy away from going for another solo flawless when asked if they only got a one and done ages prior.