Weighing them to see if they are gaining and how much is a good place to start. It sounds like they are growing fine.
I think adding weight, (barring worms or a health condition) is very easy and small changes can make a large impact fairly quickly in most cases. That is why I suggested adding only another 1/4 cup or 1/2 cup extra feed if/when you do decide to make a change. Two weeks with that little bit extra will usually have a pretty obvious effect. If I understand correctly a 2.5 to 3 on the body condition scale is actually ideal for most (open) breeding animals. I interpret that to mean pets as well.