Oh, somehow you'd know it when The One comes along. It's not
something like physical attraction. Hmmm how do I explain it?
Haha..
I think it goes far beyond that. Perhaps, being able to relate to
each other's thinking, sharing common interests etc might be some
of the factors. But no, definitely not by spotting the tattoos on
the girl/guy, do you determine if she/he is the right one!
By the way, I'm still waiting patiently for The One to appear in my
life. Haha.. no doubt, I see lotsa pretty girls around, I believe
looks can only bring you *that* far. But I think.. haha. I had
better concentrate on my studies! 
Good luck to you! Heh heh.