That would be very sneaky and dishonest. I guess you could make a test account, just add that one relay, and see if your post shows up?
Discussion
Yeah, you don't have to create another account, though. You can just take someone's public key and see the nostr universe as them. Go to Iris.to and view someone's profile. If you're using an extension to keep your private key elsewhere, it'll let you 'view as' anyone else. It'll switch to their relays and everything.