Do get a check up done as advised above. Also keep in mind that generally big dogs and specially guarding breeds do not bark too much like the small snappy dogs. The reason is, that only if there is a genuine risk/ problem will a typical guard breed dog bark to set an alarm. They generally do not bark at any and everything. This is what I have seen and experience. If there is a genuine problem they will bark and you must also go check it out, why they did bark in the first place.
One also has to look at the dog breed development/ profile to further understand the dog. The bull mastiff was not breed to go chasing robbers etc, rather it was breed to scare one with its impressive built and hold down people who used to sneek into the game reserves in the UK and hold them down till the gamekeeper arrived. You may notice the bullmastiff trying to scare people by standing or coming and then stopping, looking very impressive and scary. Its what the breed was originally breed for.
All breeds were breed for a particular reason/ work and they still retain those traits and hence it is very important to study the habits of any breed before buying one.