A common trick is to raise an exception with a failwith "not implemented"
or just assert false
.
This will give you a warning if you have unused parameters in the unimplemented function. Although, they are disable by default. So you need to add -Wall option, to enable them.
Also, you can just add an intentionally unused variable in your implementation, like:
let reverse xs =
let unimplemented = () in
xs