How did Union troops estimate confederate troop sizes?

Hello. I don't need an answer (unless you feel like giving one ;) But I am looking for where I can find the info for myself. My question is "what are 3 innovative methods the Union used to estimate Confederate troop strength?" AND Why were these methods unsuccessful (I can probably answer this once I find the 3 above) But I'm not sure where to look and I didn't see it jump out in my readings for cl. Thanks.any help would be great thanks