las has two different kinds of check-in terminals: (1) dedicated aa located near the check-in counter, (2) multi airline, you can print a bp for most airlines departing las, these are located in the middle of the 2nd floor. If you have no check-in luggage, depending on how you arrive at the airport, (2) can be very convenient, since you are close to the security lines for your gate. For aa, if you print your bp from (2), it will list only a single letter indicating your class of travel; so if you have already been upgraded to first, it will list only X; nowhere else on bp does it show "first."
The tsa people, unfortunately, don't know airline code letters; so if you want to use the fc check-in line, don't print your bp from (2).
PS: Some hotels, as well as the convention center, have type (2) terminals.
Added after my op: I re-read my bp, it did have first on it--I guess I didn't read it carefully enough. But I am certain that at one time when I checked in "first" did not appear, or was not clear enough for tsa agent to read, since he sent me to the "regular" line.