You might be able to feel the eggs as lumps in her lower third, either in her sides or her belly. Easiest to feel if you let her slide through your hand. However, not all females exhibit this.
Before laying, most females will go off their food and be restless, hunting around the tank for a laying spot. You'll need to put a laying box in the tank if there's a chance she might lay, otherwise you risk her laying somewhere inappropriate like the water bowl, or getting eggbound.
Sometimes immediately before laying they get very bloated in their lower third and feel almost wobbly when you pick them up.
On the other hand, some females exhibit none of the above signs. Apart from that, the only foolproof method of telling whether a female is gravid, is when she lays eggs!