This is a very generalized point, but typically with ASTRO 25 systems, MSI likes to configure the temporary working user ID's (WUID) for ISSI roamers in the 167xxxxx range. If you see a UID in that high a range on STARNet, it's likely coming over the ISSI or is a subscriber from a foreign system that has roamed to STARNet.
Likewise, temporary ISSI talkgroups can often be found in the 65xxx range, although all of this is configurable. Two systems I'm familiar with, for example, have TGID range 61000-65535 reserved for ISSI.