How do you know if moles could be cancerous?

I just got a red/pink mole removed off of my scalp. I had never noticed it before but my mom said i had always had it. It randomly got kind of swollen and itchy so i decided to get it looked at and removed. When i got it removed my doctor said it was benign. But i just found another similar mole on my scalp that i don't believe was there before. I don't know what to do. It is itchy and seems to be kind of swollen too. I don't know if i should be worried or not. Please, please help in any way you can.