The cheap and easy way is to assemble a set of nuts and bolts in the various sizes. In the current world though with both metric and inch threadas, it gets a little more complicated as some sizes will cross fit. I look to the equipment for a clue. Generally, inch and metric threads aren't mixed ( I know there were exceptions in the automotive industry a few decades ago). I look at other fasteners on the machine. If one is metric, they are most likely all metric and vice versa. Looking at a USA made lathe from the eighties and earlier, it would almost always be inch threads. Modern machines made overseas, regardless of whether the manufacturer is a US company or not will almost certainly be metric.