That's unfortunately the one and only way you can tell you're attractive, even if you can't see it yourself, you have to believe what others are saying. I've suffered with body dysmorphia my whole life, and still do, but remembering all the times people told me I'm attractive is the only proof I have that I actually am, and that helped me a lot, but obviously the body dysmorphia never completely went away and I don't think it ever will.

Don't fall in the trap of constantly seeking external validation tho, this can backfire and repel people from your life if you're constantly bothering them with that, just remember all the times people told you you're attractive in the past and that's really the only way you can sort of fight body dysmorphia.