If the entire rotation is a Mil charter, it will start with an M. Beyond that, you need to look at specific call signs for each leg. 8XXX is usually a charter leg. Not sure if there are other dedicated numbers beyond that (9XXX is a ferry/reposition I believe). A lot of times the layover city and dep/arr times are a good clue also. Like a 2330 dep from HPN to PHL, flight 8848, on a 757 was a hockey charter.