How to align two parallel wheels?

Not quite sure in which category it belongs. I want to built a kind a large tricycle. I can weld and I want to build it out of existing bike parts. I've got most of it together, but I am a bit stuck how to make sure the two back wheels which should be parallel are indeed so. Each wheel is in a fork and I want to weld the forks on each end of a bar which will be parallel to the floor. I think I will have to build a wooden jig. I'll probably be able to muddle through, but if somebody knows of a good way to do this I'd be very interested to know.