Is there anywhere you can actually see your overall strength? If it is, I haven't yet found it.
Pressing F5 will give some general information, including the combat losses so far. But this is rather pointless unless you can compare it with other numbers. I would very much like to see initial strength, attrition losses and actual strength included here as well.
Maybe I'm missing something, but I can see no other way to figure this out, but to make a list of all units and strengths at the beginning of the scenario and then making an overall check, corps by corps, unit by unit. We're in 1812 and attritional losses is a major factor!