Is there a clear way to tell if a phone has been rooted or not just by looking at a screen?
Yes and No, but mainly no. If you know what phone the person has, and see a custom ROM, they are obviously rooted. But there are custom ROMs that look like stock ones (Skyraider Sense for example). There also might be a root only app (Titanium Backup, Superuser, etc) icon that you see, therefore they would be rooted.
However, if you don't see these, you aren't able to tell