I can almost guarantee that a new(er, ish) shock will still be OK as far as nitrogen is concerned.
Unless someone who owned it before was the stoppie ninja, it should be OK. Also, most shops send shocks out to be serviced (more time). I would go ahead and go through the BS of mounting it. Simply hopping up and down on it whil manipulating the settings should be enough to test it's function.
If it doesn't work, at least the hard part was done.