UPS operation with a generator is dependent on how well a generator will maintain the frequency (slew rate). If the power draw is changing rapidly and the generator output frequency is not stable enough for the UPS to sync with it, the UPS will go into battery mode and eventually shut down. Most newer generators do a better job of holding frequency stable, allowing the use of standard UPS. Older generators tend to be not as reliable. There is not a way to know whether the UPS will work or not without actually testing it. For fail safe operation, a UPS with a generator mode should be selected, but they are usually more expensive.