tell (something) from (something else)
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.
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.
tell something from something
to know something because of something, such as evidence, signs, experience, etc. I can tell that she's lying from the way she holds her eyebrows. I can't tell anything from what you told me.
To perceive something as being different or distinct from something else: I couldn't tell the homemade cookies from the ones that were bought at the store.