Lust is selfish. Love is selfless. Like doesn't have the endurance of love.
I'm not the norm. I admire good looking men but I don't lust after them (meaning I won't let them touch me). Lust doesn't kick in for me until after I'm really into a guy (I have a base form of love) and when it kicks in, it's in hyperdrive. I want him and only him. The guy could be in Siberia, God could drop LL Cool J in my lap and LL just won't cut it. My body, mind and heart are three precious things to me and I am picky on who I give it to because those are the biggest "gifts" I can give. Any sort of romantic entanglement with LL (even a kiss) is not worth potentially losing someone I love and loves me back.
That's how I know. I naturally become selfless, considering his feelings first and it doesn't go away when another handsome guy gives me attention. That doesn't mean it's reciprocated and I think that's where a lot of people get into trouble. They give their heart to someone who isn't giving their heart back. When someone is really into you, their actions will match their words.
Not just talking about romance/relationships, but I've always been able to tell who I loved in my life. They're the ones who can hurt me the most.
That's the tell-tale "oh uh" sign for me. When I find myself reacting to things I normally wouldn't care about. The flip when I know I've hurt him without meaning to and I just want to make things right.