And how exactly does one look "gay"?
In porn? If you come across a thumbnail of a guy, and the dude is noticeably handsome and/or masculine, he has well defined muscles and body hair, then it's probably gay content. Or if the dude is handsome but also effeminate, then it's probably gay content.
Edit: Some examples:
One,
two,
three,
four,
five,
six. Actually, something much easier. If you can tell the difference between bara and yaoi, and see that twinks aren't really androgynous - you're probably good to go. Smut is manufactured content, the character designs and casting decisions are deliberate choices to fulfill a desire. IRL, it might be harder but not so much in fiction.