How do I ensure my baby is getting hind milk? Also advice on how to help her gain weight through breastfeeding

My daughter is gaining weight but slowly. She is 2 months today. Her doctor wants to see her back in 3 weeks for a weight check. She was born 1 month early and is 7 lbs now. He told me to make sure she's getting the fattier milk with the most calories. I let her eat on one side as much as she wants... how do I even know she's getting the fatty milk? I see breast fed babies gaining weight quickly. I do not have a low milk supply so that is not the reason. She pees 10 times a day and poops at least once or twice. I will be sad to supplement, I did in the beginning. At this point it's like we have to force her to overeat for her weight to be satisfactory. I'm hoping if I do have to supplement, that he will work with me and maybe only supplement once or twice a day for the extra calories.