How to know when weight loss switches from initial water weight loss to actual fat weight loss?

Probably after the first week. There's only so much water weight you can shed.

Measuring body fat may help:  I lost 6 lb but only 1% of fat which is only 1.5 lb 

I met my weight loss goal, and after I increased my carbs recommneded for my weight, I gained 3 lbs. in 1 week.  Is this water weight?