Option 1. Measure the distance between the lugs of your watch, where the top end of the straps will go.

Note: Make sure your watch has standard pin holes on the inside of the lugs for quick-release spring bars to latch into.

Option 2. Check the manufacturer's website, or if it's an older model — Google your watch model/year along with the words "lug width". Even better is if you can look up the exact reference number of your watch and Google the lug width for your exact reference number.