Can a serpentine belt burn up because of alternator may be bad ac compressor lock up or burn up on my 2006 Ford Focus?
Yes. If the compressor or alternator is locked up the belt will great a large amount of friction as it slides over the pulley and burn the belt.
Automatic tensioners are calibrated to provide the correct amount of tension to the belt for a given accessory drive system. Unless a spring or damping band within the tensioner assembly breaks or some other mechanical part of the tensioner fails, there is no need to check the tensioner for correct tension.