The right question is rather or not the camera currently needs to be serviced, not necessarily if it has been recently serviced! If an M camera is properly serviced and used frequently, a CLA can last 20 years or more.
IMHO, pretty much the best way to tell is to listen to the slow speeds below 15th of a second - they should sound sure and confident. Plus, the 'proper' sound at 15th of a second is a very unique "snap, tap tap tap".
If the show speeds drag or hang up, and all you hear at 15th of a second is a solid snap - the lubrication is dry and your camera needs a CLA.
Also check the shutter curtain for pin holes; this is more common than you think!