Is my cat pregnant again?

I have 2 cats that are about a year old now, its a boy and a girl (brother and sister). she has already had kittens, about a month ago she had three, and now I am worried that she might be pregnant again, how can I know for sure with...

out having to go to the doctor's for nothing?

Have her spayed and the boy neutered.
Do you realize that MILLIONS of cats are killed in shelters every year?
Why are you allowing them to mate and have litters.
Part of owning pets is to take care of them and that does not mean just giving them food, water and playing with them.
A responsible, loving pet owner has their pets fixed so that they will live longer, be happier and healthier and not bring unwanted litters of kittens into an already pet overcrowded world.