Forced labor is labor that goes outside of the normal housekeeping routine for prisoners. It's really not that hard to make that distinction, you could even do it through the bookkeeping department: any time the prison or its operators are compensated directly or indirectly by outside parties for work done by prisoners that's forced labor too.

Keeping their cell and spaces they use voluntarily clean: not forced labor, just housekeeping. Keeping common spaces clean: forced labor, the operating of the prison itself is at the expense of the state.

If prisoners are allowed to do any of that work they should be compensated at market rates to make sure there is no unfair competition.