How do I find a bolt size?

Try without cross threading or forcing a bolt into the hole screwing a bolt into the hole just to get the correct thread pitch, then use a paper clip or short screw driver as a depth gauge to find the bottom of the hole, measure the depth of the hole, the bolt needed will be slightly shorter (bolts don't go to the bottom of the threads)? Any garage will have a thread gauge to tell the thread pitch the markings on the bolt head will tell a good mechanic whether the bolt is metric or SAE thread.