I would just look at where the cables attach to the top of the carbs. You will be able to feel any slack in the cable with your fingers and can adjust it there accordingly.
Just make sure the carb is completely shut when off the throttle, and if you make them too tight the throttle won't return.
You can probably get to the adjusters with the tank off and by displacing the bracket where the coils and fuel pump are attached.
It is a bit of a faff.