Inspired by @Nucejoe's method I will try to test the slippage of my mainspring like this: According to the specs the power reserve of my movement should be 40h. The number of leaves on the centre wheel pinion is 12. The number of teeth on the barrel is 84. For the centre wheel then to rotate 40 turns, 480 barrel teeth is required (12 leaves x 40 hours = 480). 480 / 84 equals approx. 5.7. That is, after about 5.7 turns of the ratchet wheel the mainspring should slip. If it slips sooner (which is the likely scenario) it won't yield the 40 hours and I will have to take some action.
Big question though; will it really be possible hear and/or feel when the mainspring slips? What's your experience?