jb55 on Nostr: One way to do it (but its not currently that common) is to check the relay field of ...

One way to do it (but its not currently that common) is to check the relay field of the user's nip05. I added this to nip05 for this exact purpose. Then all you need to do is get the user to insert their nostr address to login.

Another way is to get the user to insert their nprofile when logging in, which provides the client with a list of relays to bootstrap from.

Some clients ask the user to explicitly choose relays they would like to bootstrap from. I find this not very user friendly so I don't do this method.

Some clients use a fixed relay list to bootstrap from.

Many different approaches you can take.