Look at the ends of the broken halves. An over torqued fastener can show signs of the stress as the metal is partially rotated at the break point. A swirl pattern in the outer regions of the broken face is a good indicator of over stress. A shear failure without over-torque will look much different as the bolt simply snaps off in one direction often leaving a rough protrusion on one side.
Without some signs of corrosion, a shear failure is unlikely unless the metal was fatigued from over torquing or an incorrect fastener was used (e.g. material or hardness grade).
You might also want to measure the length of the end that was in the block and then the depth of the block hole. This will tell you if the correct bolt length was used or if the bolt had bottomed out prior to proper torque on the tensioner assembly.