That's the best thing to do, if you look at it off the bike it will tell you lots more than when it's hidden behind loads of gubbins. Measure it as well and compare that against a genuine shock, if it's much bigger than standard then there's your problem, a second hand shock is not always the best way to got, you never know how the thing has been treated, a new one will set you back a large chunk of change but then it will outlive the bike plus it's easier to sell if you tell folks that it has been done.