Had a cutlass with a stuck exhaust valve found by pulling the prop through. There was no mistaking it - no resistance. Engineers flew up and the did the 'old rope trick'.
When pulling props though always ensure:
Mixture is in idle cutoff
Throttle is at low idle
Mags are off
Brakes are on, chocks are in, and preferably tied down as well.
Always, always pull in a manner and from a position that if it starts the prop isn't going to touch you.
No loose clothing (ties etc)
From memory with the 172 RG at TDC the propeller is at a very difficult position - not an aircraft that I'd be in any hurry to hand start.
One engineer I know is very definite about the direction of rotation - always backwards - as a result of having seen someone cut up by a prop.