You can do a visual check to make sure there's no leaking, but Ford/Fox recommends rebuilds every 50k max with 100% on street use. Some rebuilders recommend them to be rebuit sooner. I mean you could push it further, but risk damaging internals.
There's a chance the previous guy could of rebuilt them, but no easy way of knowing unless there's a different decal than stock.
https://www.ridefox.com/service.php?m=truck
As for the other comments, here's a good link to follow.
http://www.fordraptorforum.com/f4/buying-2nd-2010-14-raptor-first-46952/
