Getting weight and wing measurements of adults can get you a reasonably (90-ish %) accurate idea of gender. For babies, they'd have to check chromosomes. With adults, you can see if they lay an egg or develop a brood patch, and some will do other gender role type activities that can help distinguish them.

A handful of owls will have some visual indicators, but they're not always very accurate. Barn Owl females usually have more belly spots and a browner "beard", but it isn't too difficult to find examples that break that guideline.