"How do I make sure my boots fit before I try them on a trail? I don't want to return used boots." I can relate, here's what I do.
Try them on at the end of your day when your feet are their largest. Wear the insole and socks you expect to use on trail. Walk around in each indoors for at least 10-30 minutes, the longer the better. Ask yourself which shoes are more comfortable and why. Can you make them more comfortable? Adjust fit with lacing, tongue, tightness? Keep wearing them indoors until they are either "right" or not right by gut feel. Heels should stay in place securely. Toes should be away from shoe inside perimeter not touching anywhere. Your foot should feel it can spread out and that it is not constricted in any way, sides included. There should be an overriding sense of comfort everywhere. If not, keep looking.