The person needs to be directly involved in a crime.
If they are dealing drugs for the gang, and that link can be made, those charges will stand.
However, if a kid wearing Crips colors is walking down the street, minding his own business, with no drugs or weapons in him, and not a suspect in a crime, he is not going to be convinced of belonging to a street gang.
Yep. But we don’t bother law abiding people. However, once we validate (a serious process), that “kid” (or 35 year old) can be charged.
You asked how to know and with what charge...
I am the how. I supplied the charge. Now you know.