As brief background: years ago I was obsessed with getting a 6-pack. I was possibly the worst body type: skinny but with a flabby midsection. After years of P90X, I didn't quite achieve a 6-pack, but I did get a nice 4-ish pack; I still had to flex a little to see it on my skinny frame, though.
Now: I feel like I'd like to gain some muscle mass. I've read from various sites, including this StackExchange, that when adding calories to fuel muscle growth (along with exercise, of course), it is inevitable that you will add some fat, i.e. your gained weight cannot be 100% muscle.
Question: I plan on ramping up my caloric intake, perhaps in 100 calorie increments, while doing a muscle growth-themed workout (Body Beast). How can I know whether I'm getting sufficient calories to fuel muscle growth? Although I cannot avoid some gain in fat, I would like to minimize it, which is why I am trying increasing my calories in 100 calorie increments.
Also, what would be the long-term (weeks/months) effect of performing a muscle growth-themed workout without ingesting sufficient calories?
(Btw, I know gaining muscle mass is a years-long endeavor; i.e. though I'm doing Body Beast, I know to not expect drastic changes in 90 days)