For me, and this is only for me, sometimes a fabric will just "capture" me and I know that If I walk away, drive away or try to be brave and not purchase it, that I will regret leaving that fabric behind. That is my signal that I really like that fabric and I use what I am drawn to. It has happens from time to time that I will walk away and then days and weeks later, that little beauty will still be lurking in my head and I have the perfect project in which to use it. Often when I get this "gut" feeling about a fabric, I will wander about the shop for 10 minutes or so. If after that time that piece is still calling my name or just lurking in my memory, after seeing hundreds of other choices, I buy a yard or two or more if it is a bargain. If I like it that much, I know I will use it.
The opposite happens too. I see a fabric I like but after my 10 minute wander I can't remember where it is or I am feeling others are just as nice, I skip it, even if it is on sale! A sale is not a bargain if it just sits and is not used.