Take a thin solid object – like a chopstick.
One you make black (for bright daylight scenes), the other you get some fluroscent, bright colour spray for (dark late stuff).
Then when in doubt, you take a look from the front of the camera, place the stick horizontally accross the line of gradutaion (where dark bits become lighter) against the filter, and holding steady, slip you head back behind the viewfinder to check if it’s in the place you would like it.
If it’s not adjust.
Techniques differ for the grads which are held in place, or the ones which are screw on filters.
With the screw on’s it’s nearly impossible to get the graduation spot on – trust me: I have one.
My chopstick tip is very unrefined, I am sure some other members could come up with someting much more ingenious.