My IDE highlights the word in yellow (warning), and hovering over it produces the following pop-up: Unable to find sub definition, declaration, constant definition or typeglob aliasing. It also throws a yellow dot at the line where the warning is located, so even if that line isn't in view on the screen, I can always see if there are any warnings/errors anywhere within the file at a glance. Everything happens live-time, so I can actually see it immediately after I'm done typing.
Here's an image of what I'm speaking of. If I point at the word, the notice pops up.