1. To be able to discern or distinguish some thing as being distinct and unique compared to something else. It's very difficult to tell the real paintings from the counterfeits, but there are a few things we can look for to be sure. Because of my color blindness, I can't tell shades of red from shades of green. The princess dressed so plainly that you couldn't tell her from an ordinary citizen.

2. To be able to perceive, recognize, or understand something from some piece or amount of evidence. In this usage, the noun or pronoun between "tell" and "from" is sometimes relocated to the end of the clause. What's wrong? I can tell you're upset from the tone in your voice. Well, we can tell from these markings that the herd is moving towards the north.