You will have to check the hubs and see what style they are.
If they are a large hub with 4 bolts you may have this style wheel.
http://recstuff.com/trailer-tires-wheels/trailer-wheels/14-trailer-rims/14x5-5-4-bolt-on-9-44-vintage-silver-trailer-wheel-5mm-offset/
If you have a standard small 5 bolt hub you will have to measure them to see if ther are "5 on 4.5" or "5 on 5"
here is a guide to measuring.
http://recstuff.com/pro-tips/measuring-your-wheels-bolt-pattern/
5 on 4.5" would likely be a 14" wheel
5 on 5" would likely be a 15" wheel