Using a thread pitch gauge is the best way to determine threads. If you can't find a metric pitch gauge then the next best method is using your bolt. Some, not all auto stores stocks nuts and bolts but you can try. Home Depot and other big box stores have nuts and bolts too and you may find sizing info there with your bolt.
Use your bolt and mesh it to another one of the same diameter. When two bolts mesh completely together, you matched threads. incorrect threads won't mesh. Use the size associated with the bolt size, the second number is thread pitch; 1mm, 1.5mm, etc. The first number is the diameter of the bolt threads. You can put your bolt in front or behind the store bolt and see from about two feet away if the store bolt is the same diameter, smaller or larger. Your micrometer caliper is great for measuring diameter. Diameter x thread pitch determines the tap size needed. Then decide if you need a starting tap (tapered) and/or bottom tap (tapping a bind hole).