As MEL says there is a mark on the shaft adjacent to where the #1 blade goes. They are then numbered CW aft looking forward. So looking into the engine from the front anti-CW.
BTW. Do not assume numbers written on blades correspond to their position as per the AMM. The numbers may have been wriiten to aid blade removal and re-installation and may bear no relation to the actual blade number.
This convention applies to GE/RR etc there may of course be other numbering conventions for other types