Coming back from Ireland I felt that the tabs where adjusting them selves so I kept adjusting so the question is how can I test them and is there a retro fit position indicators
Not sure about electronic position sensors but (morse) cable driven ones would be a doddle to retrofit. I have some on mine which you can have a peak at.
To test them, put them down, run some miles, don't touch the controls, and check at end of day whether they have come back up
Retrofit indicators are available for some makes. depends what model you have. Full details on websites for Bennett and Lenco. You can also fit mechanical indicators using Teleflex/morse cables but that involves potentially a big job installing the cable or perhaps an easy job- it depends entirely on the particular boat
What Brand of Trim Tabs do you have. If they are hydraulic, put them down all the way and leave them there over night. If one or both come up on their own (and there are no leaks) than one or both of the valve seats are leaking.