I think the best tell-tale sign is when one or both of you cease to view each other as priority to each other. Other interests - friends, a hobby, etc - take precedent over you and consume an excessive amount of time that you would have otherwise spent with each other.
There is always someone who wants what you have. Either of you conscious of that fact brings in temptation to "taste" the water. True love for one another would never succumb to such a thing.
Respect for each other without desiring to control and emotionally hold each other hostage must first exist before a relationship is solidly grounded.
Serious self examination of ones own feelings (and lusts) must be conducted. The question one must ask themselves: "Am I ready and mature enough to love another? Does the other person possess that same level of maturity to be in a loving relationship with me? Do I do things to make others jealous? (emotional blackmail which is dangerous). Am I the jealous type?
When looking for a person to be in a loving relationship with you, you MUST look beyond the "external" appearance. Sadly, many look only there. Look to what a person "proves to be" on the inside. That takes much more than a one-night stand.
It's a gut feeling I get through years of experience. Sometimes they out grow you or they realize they were never in love in the first place.
Well for me it was that whole coming home & putting out a fire on the lawn, only to discover it was all my clothes burning....lol Just kidding.
Seriously though sorry to hear of your situation. I hope life gets better :-)
"Falling in love" only lasts a short time, never more than a year and a half at the most. It's like the life of a butterfly, beautiful, precious and very short. Real love happens to people after they are past the falling in love part of their relationship. Strong feelings do not equal strong love nor lasting love.