Symptoms of mites on guinea pigs?

i think my guinea pig has mites. What are the symptoms, and what meds do i give her? She is a white crested FAT guinea pig, and is about 1 year old. Any symptoms, and what meds do i give her? I cant afford to go to our overpriced vet, and the most i can afford is about 15-20 dollars of meds. Thanks!