Listen for the announcements. The US BP only lists numbers for zones, so anything that would be preferred access is zone 1. This includes First Class, DM Elites, Star Elites, AA Elites, etc. All will say Zone 1.
The most usual process in the last week is pre-board special needs, invite to board all First Class, followed three seconds later by calling all Uniformed Military, US CP, AA ExPlat, followed almost immediately by the rest of the elites, then the zones.