Nope. A guy who is in love with you WILL look at you in a special way. But a guy who wants you to believe he is in love with you will look at you the same way. Sorry. It's not magic, it's a look-just a particular facial expression. If sincerity vs. deceit could be determined just by looking in someone's eyes, surely there would not be so many successful cons and cheats? Of course we like to believe we will "just know" the truth, but in the end it comes down to trust and faith, not any sure-fire test we can perform.
Also, there is no such thing as unconditional romantic love. Romantic love is hard work, two people dedicated to the cause of making it last and grow. Everyone has their point of no return, like many people could never forgive an affair and most people could not forgive physical abuse, but even people who could forgive those things have SOME point past which they will not go. Ever heard of a wife putting up with years of abuse and then leaving the day the father finally turns his fists on their child? Even a person who has been demoralised and broken and accustomed to the very worst treatment and still continued to love the creep, there is still a line in the sand. In my experience, people who want to believe in "unconditional, forever, no matter what" love do NOT want to put the effort forth to actually have a sustained real-life love.