2 ways to tell easily.
The first like Leonais said is when you are making a new game your starting companion will have an option as a "Keyholder" below the initial 4 options (childhood friend/stranger/etc)
Second way is there will be new sex actions when in a sex scene with anyone, mostly in the "S&M" tab, new actions are "Slapping", "Ice Genitals", "Numbing Cream", "Orgasm Blocker", and "Lock into Chastity".
If you aren't seeing this you may have not activated the mod in the mod menu.
Main menu -> Mods -> Ensure "ChastityBelt" is activated (orange) -> Click "Apply" -> Wait a second or so -> Close game and reopen.
With mods mods you only need to apply once, ChastityBelt is an exception and you'll need to "Apply" a second time for all functionaly to work. If you've only applied once you'll get a warning when you go to your Mansion.