OK, if they are standard, you need to count the threads in one inch. Start from the first thread and measure an inch and count to that thread. With numbers that close, it may be difficult to determine. If you end up with 19 threads, it will be hard to decide. Make sure you measure carefully. If you can remove the wheel to get more length of the stud to measure, like 1.5 inches, you can count, then divide by 1.5.