Web of trust.
For instance, if more people I follow and people they follow are already following ODELL (nprofile…p7e2) instead of ODELL (nprofile…acus), then I can be reasonably certain the first is legit and the second is an impersonator.
Start building your web of trust from people you know outside of Nostr and it won't take long to distinguish between most folks that anyone would try impersonating.