They might be self-pollinating; I think some citrus plants are.

Otherwise, if they're a size that you can reach, you can pollinate by hand. Get yourself a small feather or a soft paintbrush, like the kind that comes with a kid's watercolour box. Starting at the top (or wherever; it's just for ease of keeping track of where you've been), sort of "fluff" each flower to transfer the pollen around.

If these are big trees, well... I have no idea, but you could at least reach the low branches?

Good luck!