The thumb trick is a way to know if one will pass over or under cloud thst is say ten or twenty miles away on ones track; going through is not an option VFR and one can never be too sure how deep it is.
It is fairly simple and worked very well on this trip as the air was very, very smooth.
So, thumb trick, close one eye hold out arm, steay hand on glare shield, line up top of thumb with top or bottom of cloud, hold still - if held on top of cloud and the top of cloud seems to be creeping down one will go over the top. Similarly, if the top of the cloud creeps above ones thump one will not be passing over - in which case do it with the bottom of the layer to see if one will pass under.
In my case I was about 120kts in the cruise and steady at 5,500ft - the cloud layers generally were about bottom 3,000ft and tops 5 - 6,000.
This trick is something I saw mentioned by someone on this board a few weeks ago, this trip was the first time I'd used it - it worked very well, only took about a count of 5 seconds to get a 'fix' as one gets closerone gets it even faster.