This could potentially be abused, but you could actually try to resolve the DNS to determine if it's valid (could be weird for some cases like localhost or IP a

This could potentially be abused, but you could actually try to resolve the DNS to determine if it's valid (could be weird for some cases like localhost or IP addresses). Or just do a "curl https://whatever.com https://whatever.com" and see what happens (assuming that all of the websites are running a webserver, although idk if that is true in your situation)