They give constructive criticism.

They prioritize you (within reason). As you get older, you’ll realize a lot of your friendships are fueled by convenience. As that convenience disappears and people’s lives start to get busier and farther away, you learn to recognize just how rare and special it is when someone goes out of their way to make time for you.

They initiate the reaching out every once in awhile.

At least, that's what I imagine it must be like.

They are receptive to constructive criticism.

They follow through on what they say, such as offering to help out and following through with plans.

They are supportive of positive things.

They acknowledge any self destructive things you might be doing and offer support in addressing them.

None of these need to be 100% of the time because things come up and sometimes they can't help you with every self destructive thing, but doing these things in general is a bunch of green flags.