You will need to measure the outside thread diameter to know for sure. Using calipers or a micrometer, it should measure slightly less than the decimal equivilant. 3/4 (.750) could be as small as .740. You will then need to count the number of threads per inch. 3/4-10 (10 threads per inch is coarse) 3/4-16 (16 threads per inch would be a fine thread). There is an almost infinite number of thread combinations that can be machined. I would guess that you have a typical fine thread (or SAE) as some would call it.
I would have to agree with Barry. It is unlikely that it is a pipe thread, as this type of thread is designed to bottom out (on the taper) to form a seal.
J Lucas
