Not just on a sunny day, and don't forget to check the downspouts, too!
We're finally getting some rain, after about a month of beautiful dry weather.
Being the anal-retentive home owner that I am, I took a quick walk in the rain to make sure water was only coming out of all the downspouts. In heavier rains, it doesn't take much to clog a gutter. I know this first-hand, and have had the water in the basement to prove it.
Upon checking the downspout by the tree I recently did a bunch of trimming on, I noticed that the downspout (which drains directly under the tree) wasn't draining any water. None. Zero.
The other downspouts had a little trickle coming from them, so I knew something was up.
Evidently, when I was pulling the cut branches out from under the tree I disconnected the downspout from the vertical run that goes up to the gutter. When it was re-connected, it was put on upside down which left a 1/2" gap in the bottom (hard to explain, but trust me) that all the water was pouring out. Right up against the foundation.
Not good, if the rain were to continue for too long.
5 seconds later and it was fixed.
No damage, but if I didn't notice it and it was left this way too long during a storm I could have ended up with a lot of water in the basement.
Moral of the story: check the gutters AND the downspouts. Not only when it's nice out, but when it rains, 'cause that's when you'll notice if it's working or not.