Metric threads are expressed in pitch. Imperial threads are expressed in the number of threads per inch. This is their biggest difference. Metric threads are 60° equilateral thread types, with outer diameter and pitch in millimeters. Imperial threads are 55-degree isosceles thread types, with outer diameter in inches. When reading the outer diameter with a metric caliper, there are usually decimals. Sometimes, the BSW logo will appear, representing the imperial connector.
When there is no thread gauge to measure the pitch, you can measure the outer diameter of the connector with a vernier caliper. For example, one measured outer diameter is 22mm, and the other is 21mm. Through the query, it can be found that the 22mm is a metric connector with a pitch of 1.5. While the 21mm is an imperial connector with a pitch of 14.