Give the linesman three flags. A red one for definitely offside, an amber one for he might be offside I'm not sure, and a green one for he's onside.
If appropriate two flags can be raised at once eg red and amber means he's definitely offside, I think.
Not raising a flag is not allowed. Any time a forward pass is made which could result in an offside call a flag must be raised.
Having an amber flag raised at any point will cause a VAR review next time play stops.
Having a red or green flag raised at any point and having it be the wrong decision means no VAR review but the linesman is taken from the ground and shot so he can be replaced by a more compliant linesman who knows his place and uses the amber flag. All hail our VAR overlords.