Locate an AC current leakage either to safety ground wire or
via some wire (inside wall or in appliance) to earth. The
current leakage is something greater than 150 microamps.
That's right - micro. All appliances have some leakage. So
when the GFCI trips, the leakage in microamp range increases
to the milliamp range. Find the microamp leakage and you may
find the milliamp leakage. Should current leakage be in upper
microamp range when GFCI does not trip, then suspect a 'usual'
suspect.
Obvious and not waste time: you require both good eyes (the
visual inspection) and something that provides numbers
(ubiquitous 3.5 digit multimeter that can measure low, AC
current). A neutral wire (after GFCI) too close to a safety
ground wire can leak current in damp weather.
Generally something that trips during rain would also be
leaking 'less but too much' current during the 'dry season'.
There is more to this. But the point is to ask whether you
are ready to attack to the problem - or hire someone else. If
you do it, then it will take much longer. But you will
learn. The 'hired gun' costs less because he solves it
faster. However then you have learned nothing. Your choice.