Even though our shelter does not guarrentee that the dogs adopted will not be aggressive- they do however keep them them for 5-7 days to determine if it is likely.
I have had several shelter dogs. The only thing I have seen in general with mine were lack of training, lack of love and understanding, and general socializing. I will say all 3 housebroken very fast- its like they really knew the difference from outside and inside. Also after training, love and a secure place to live, they became the most affectionate of any of the dogs.
I would suggest taking a trainer with you to the shelter if your mother is really concerned about it. Even that is a streach though. Shelters will even euthanize a dog suspected of aggression.