Interesting skies can appear at any time of the day or year. Common times that skies are interesting tend to be around sunrise or sunset, or when interesting cloud formations appear. Combine the two for a really punchy sky.
Keep an eye on the weather
The best way to judge if there's going to be an interesting sky is to look for yourself, sticking your head out of the door or window to keep an eye on things. If an interesting sky forms while you're out, set up and see what happens, you might be able to get some really great shots.
Which Tamron lens should I use?
A wide angle lens will be ideal, such as the Tamron 24-70mm f/2.8 VC USD. This will enable you to make the most of the sky and capture it in all its vastness. Make sure you pay attention to what's happening at the edges of your shot, as other less interesting elements could creep in.
A tripod will be ideal as you might be shooting in low light conditions if your interesting sky appears at sunset or sunrise. An ND grad filter will help to balance the colour of the sky and the land to give you a more even exposure if the sky is much brighter than the land, and a solid ND filter will help to pop the colours if you're just shooting the sky.