A dog came to my door and wanted to come in what do i do?

My mom let her in and said we could keep her since she got along real well with my other dog she is very good on a leash and it seems as if she has been trained before but when she came she was hungry thirsty and was dirty she is in good health but has a marking (?) or a dirt spot on her side and on her tail it does not feel the same as her other fur but i gave her a long bath and she did not resist much. so its like to questions.Also we have no money to take her to the vet but like i said she seems to be in good health. :) thanks