I have a ‘trained eye’ but it’s still a guess. How long is she? (length). The bump squares off at the front when she’s about ready but young ones don’t square off as much. It may be starting to square off, in which case I’ll guess about a week, or if it’s one of her first few batches she could be squared off already and even imminent...
Just like already mentioned before, even a trained eye is not always able to make a close estimation. Round or square shaped are two different things. A guppy female that is almost due can have a round or square shaped belly. So, she doesn't have to square off to drop fry. But when she's squared off, it only means that she's used her max gestation period before dropping those fry.