First, take a look at the serial number (typically on the stern) and make sure they're selling the year they say they are. Meaning if it's a 2013 the last two digits will be 13; Read more here
Flip it over and take a good look at the hull. Scratches are OK, but deep gouges could cause future problems. Great thing about the Ultimate is you can see both sides of the hull easily. Bring a flashlight to light up the hull if you want.
Make sure there are two removable metal cross bars. If it's the solo version the foot braces are fairly large. If it's the tandem there will be one large set and one small set.
The foot braces use a rod that when you apply pressure allows the foot peg to adjust forward. When you pull back it moves the pegs closer to the seat. The spring mechanism on the older models would wear out some so when you apply pressure the foot pegs disengage and slide forward. So sit in the kayak and apply some pressure to the pegs to make sure they don't give.
Other than that they are really simple, straightforward kayaks.