There are many types of pipe threads used here in the USA. The most common and easily found at the local hardware store is NPT. This is a tapered thread and is most easily checked by measuring the outside of the pipe or inside of the inernal thread and then checking against the following:
NPT -- OD of pipe
1/16- 27 -- 0.3125
1/8-27 -- 0.405
1/4-18 -- 0.540
3/8-18 -- 0.675
1/2-14 -- 0.840
Size would be the closest/next largest.
More info can be found at
http://www.mcmaster.com/ and searching 'pipe threads' , click on 'pipe measurement'.
If all fails take the fitting to the hardware store and they can tell you by matching. If you have a straight thread then this is another can of worms and they will not match with anything you have from FIRST.