Most apps have foss counterparts but some essential ones like bank apps are only on google play.
How can I tell in advance the apps I need from Google store will work on Graphene?
Graphene as Android replacement?
You can't for sure.
If its not a popular app, then you can search around and see if anyone else has used that app on Graphene and if it has worked for them.
Otherwise you just have to try it yourself. Graphene has the ACTUAL Play Services. So most apps really should work. But some difficult apps may not (some banking apps).
If you are lucky enough to have a friend with GOS then maybe they can try installing it to test for you.
The huge majority of apps work flawlessly with GrapheneOS. And the link provided gives you most of the answers for banking apps.
@d4f2 had to remove your reply because one of the cited sources is notoriously unreliable and more likely to mislead than assist someone who's looking to see what's compatible with GrapheneOS and what's not.
@Hansel If you're looking for banking apps specifically, take a look at this list:
https://privsec.dev/posts/android/banking-applications-compatibility-with-grapheneos/
This is a community maintained list of apps that have been reported working. An app not being on that list doesn't make it incompatible with GrapheneOS. It just means someone hasn't added it yet. Realistically, the only reason why a banking app wouldn't work is if it requires a Google certified OS, and in my experience, most don't.
For general app compatibility, you really don't have to worry about it. GrapheneOS has great app compatibility thanks to Sandboxed Google Play.
If you have any specific apps you'd like to confirm, you can leave their names here and someone can try them and see if they work on their end.