I've made these kinds of repairs, so I can give you a quick tip, but don't think that I'm especially vigilant about checking my own leaking downspouts. I'll fess up -- I tend to react to repairs rather than anticipate upkeep for my home.
In other words, when I notice the busted joints in my downspouts beginning to pour like Niagra Falls, I'm pretty sure I need to fix them. Pronto.
Here's how you can be more proactive.
The problem of leaky downspouts (vinyl, PVC, or aluminium) may be linked to one or more of several contributing factors.
Gutters choked with leaves, pine needles, or twigs retain water, and wherever you have standing water, you have pressure and potential warping of the downspout joints on your drainage system.
If you've done any exterior work on your home recently -- such as redoing a roof or painting the house -- the downspouts could have been bumped or dented and the joints shaken loose in the process.
Downspout joints can, of course, also simply wear out with weathering and time, too.
Before you attempt the repair of the leaking joint, first make sure that you have cleared your gutters and fully flushed the debris from the downspouts with a garden hose.
Also inspect all of your downspouts for evidence of loose joints, where other leaks may soon occur, and be prepared to make repairs at one time. This will save you from having to make repeat repairs -- often at inconvenient times, like when it's pouring cats and dogs -- as other joints may fail suddenly and spring leaks.
Remove any securing screws, nails or straps at the joint on the downspout where the leak is occurring, and separate the leaking section from the main downspout.
Carefully clean any residual caulk or sealant with a wire brush on both the leaking section and the main downspout.
Add a beading of fresh caulk to the leaking section, and reattach it to the main downspout with appropriate connecting hardware.
Now when the heavy rains come, you won't have to worry about leaking downspouts pooling water at the foundation of your house -- and causing another kind damage that may test your upkeep attention.