I use the Hornady OAL gauge with bullet comparators to measure seating depth. A less precise way but one that works just fine for typical hunting needs is to seat a bullet in a dummy case, pull the bullet, then re-seat the bullet at a depth longer than needed, chamber and close the bolt. You can do that several times and compare values until you get a consistent result. You will see marks on the bullet where the ogive contacts the rifling. There is still sufficient neck tension to not pull the bullet out when you extract the dummy round. It may pull it slightly so this is why you need to do it several times and compare values to make sure. I would do this with a fire formed case that has been full length resized (unless you don't resize your cases) to ensure you are getting the best possible result.
