1. Just have a mixed (various troops) stack lead by a "forced march" general.
2. Let the stack 'sit' for one turn to make sure everyone is in the stack at the beginning of the next turn.
3. Check and note the movement distance with the general in the stack.
4. Remove the general and check and note the movement distance for the remaining stack.

If there is difference between 3 and 4: forced march was in effect for the whole stack. If there is no difference: alas, the feature is bugged. Unfortunately, I cannot test right now.