Assuming you're talking about a knife blade:
The best way to tell if your blade is getting dull is to use it. Over time, you'll get a feel for how sharp your knife usually is, and when it starts to feel less sharp, that's a good indicator that it's time for a touch-up.
You can also try the paper test: hold the blade at a 20-degree angle to a sheet of paper and slice downwards. If the blade catches or tears the paper, it's dull and needs to be sharpened.
If you don't have any paper handy, you can also try chopping some soft vegetables like tomatoes. A sharp knife will make clean, even cuts without crushing the vegetable; a dull knife will tear or crush them.