Do you have any other chickens you can feel for comparison? I find that feeling a healthy chicken is one of the fastest ways to tell if something is normal.
The neck of a chicken can make an odd sort of lump, and the crop makes a lump too. So those are at least two normal things that might be making the lump.
If you really cannot tell, watch how the chicken is acting, then pay attention over the next day or two. A crop gets more full when the chicken eats, and less full when the chicken digests the food (especially during the night). Checking for those changes is a good way to be sure where the crop is.