A crush is a crush, and lust is lust. Sometimes they happen together, sometimes separately. I think you can usually tell in yourself whether you are lusting or not. When you think of the person, are you thinking of him or her as a person, or are you thinking of his or her body? Are you thinking about talking to the person and doing things like going biking or going to dinner or whatever, or are you thinking about having sex with them?
Sometimes crushes are purely physical, and then I think they easily veer off into lust. A lot of times, they are about spending time with a person, and are less likely to occur with lust (although sometimes even this kind can fall into lust).
Just MHO.
–Jen