How would I know if my cat is pregnant ?

My cat escaped for like a month .. A week ago we found her ... We took her to the vet to check if she is disease free. The vet then said she's supposed to be on heat now, or she is ready for pregnancy, we took her to another vet to breed her,but because am a new pet owner i didn't really know that the cat is supposed to be vocal to be on heat, the other vet told us to wait for a couple of days to see if she'd get on heat, and if not, there is a chance that she is pregnant, as her nipples are pink and busty, but the doctor said she could have been pregnant before and that would explain the nipples. My cat was really thin when i found her, she's eating really well now, i noticed that she gained a good amount of weight in this past week, and that she's very lazy and she spends most of her time sleeping. Does this mean she's pregnant ? Or is it normal because she was out for a lot of time ? And if she had kittens, what should I do then ? If i wanted to sell them, How old should they be ? And Where should I go to sell them ? Must i go to each pet shop and ask if they need a kitty ? Or is there an easier way ? My cat is a pure siamese, so any tips on the price i should charge ?