a thread pitch gage. it has individual fingers ill call them like a feeler gage in a way with the the thread pitch number on them instead of the decimals on them.there are little pocket size charts as well that have thread sizes and tap sizes as well.anyone that sells machinist tools should have them.the chart and gage go hand in hand.the pitch gage has the thread shape right on it lay the right one on the bolt and it will tell what thread it is . pretty straight forward.the traded pich gage /plate works well too as mentioned.