How do you check the slack adjusters condition in commercial vehicle?

that's easy if the slacks are working they will adjust the brakes .if they are not working the brakes will be out of adjustment.

I work at a class 8 truck dealership, there are 2 types of slack adjusters, auto and manual. If you have auto slack adjusters, they will adjust themselves to a point, however, if you have no braking capability to speak of other parts are probally wore out, s cams or clevis posibbly s cam bushings, brake chambers. Manual slack adjusters should be checked in my opinion only at least at every pm or when braking capability suffers. Those big trucks take a lot more time and space to stop and their brakes had better be in good shape or dot will shut them down if caught.