You can climb a ladder and inspect your gutters, but you do not really know how they work until you see them stressed. The next time the rain falls, take an umbrella and go check it out.
It does not matter how much you look at your gutters, your downspouts, and your surrounding water limitation systems on a sunny day – even if you use them to test the flow – it does not compare not a check of system operation when rain falls on your roof at the monsoon.
Next time it will rain, but not lightning! Safety first! – go out into the yard with an umbrella and look for the following items:
gutters: Is the water contained in the gutter or does it overflow partially or totally on the side? With the exception of truly massive rain storms, your gutters should be able to withstand heavy rains.
Seams gutters and downspouts: Does water run over joints and downpipes without leaking?
Descent flow: Does the water come out freely from the downspout?
Flow zones: Once the water comes out of the downspout, where is the water going? Does it sink into the ground at least several feet from the house? Does it flow? Return to the foundation?
When the gutters are dry and you're just trying to get a close look at how the system works, it's too hard to really see how it handles heavy rains. However, when it really rains, you can see all the problem areas: the joints that need to be sealed, the places where the gutter level is turned off and that flows down the downspout, where there is a bad quality near your house water against the foundation.
Write down the problem areas and, if it is dry and safe, correct the problem or consult a professional to solve it for you.