Pull the belt cover off and watch as you put the wheels down. Does the belt slip on the pulley or not when the wheels don't drive.
It is kinda pain to do that with this mower cuz if u take the front apart the belt is loose so u r just eyeballing it if the belt is loose.
If u want to check if the belt is tight I think the best place is to look underneath with someone or held with a rope to see if the belt is tight.
By the handle where the cable comes out of the box is a screw. It is like a 10 speed bike- unscrew it to make the cable longer thus belt tighter. Mine went out pretty far so I took 1 more step.
hacker suggestion:
At the front if u take apart the front (8 screws I recall) there is a bracket holding the cable casing still. I took that out and put a washer in to make the cable casing longer thus the cable tighter. That worked for me
If after doing this and the belt is still loose maybe u got 1in too long....