For instance, if more people I follow and people they follow are already following nostr:nprofile1qqsqfjg4mth7uwp307nng3z2em3ep2pxnljczzezg8j7dhf58ha7ejgpz4mhxue69uhkummnw3ex2mrfw3jhxtn0wfnszynhwden5te0wahhgtn4w3ux7tn0dejszxthwden5te0wdjkuerfwshxummnvekxzun99e3k7mgrcp7e2 instead of nostr:nprofile1qqspvxckleleljlh96s2j7nmsxxecwlutwpgqpztclp7z22k8jwqedcprpmhxue69uhkv6tvw3jhytnwdaehgu3wwa5kuef0glacus, 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.