Is there a way to determine what your muscle gain will be?  Are there some assumptions that can be made, that if you do "x", your result will be "Y" in terms of muscle gain?

I'm currently eating around 1900 calories a day, doing a full body workout 3 x a week and walking for 1/2 an hour for IR 3 x a week.  I've been averaging about 1 pound a week in weight loss.  My belly is flatter and my shape is changing, so I'm assuming that most of the weight loss is fat.

Is there a way to tell, or do you just measure after the fact and that's how you find out if you gained muscle?