When you look at the save it should be listed next to the date of the save.
Well, that might not be completely accurate - if you have ever died or reloaded for any other reason, the time you played between when you loaded and re-loaded is lost.
So, let's say I play the game for an hour, save, then play for another hour without saving, and die. My save only shows I've played for an hour. I reload that, play for another 1/2 hour, then save again. The new save shows 1 and 1/2 hours played, but I actually played 2 and 1/2 hours.