A quick question about non-reving on payload optimized flights...

How do you know if it's payload optimized prior to showing up? My family tried to non-rev on 1468 SAN-CVG last night the flight had tons of seats and they left non-revs behind. This morning flight 9862 shows over eighty seats available but they have such bad luck out of SAN that I'm not even sure they should try. Are the 320's performance restricted due to the short runway with 4 hours of gas?