This is more of a best practices question, but it's very important to me because it caused me many times to think that my js or css modifications weren't working.
I work with PHP and have my CSS and JS in separate files. Sometimes when I hit the circular Reload button, my changes don't kick in. I use either Reload or re-hit Enter after the URL. Sometimes neither works, and I have to do crazy things like restarting my browser for caches to clear.
To test whether I'm working with the old or new JS for example, I use different alert messages, and that's when I find out it's still using the old (pre-recent-change) JS. Same for CSS, I use different color borders, etc. to know if it's picking up the new. Needless to say, I'm sure there are better practices than this :)