I know it when I see it

"I know it when I see it" means that it can be hard to explain what something is, but you'll recognize it when you see it. For example, you might not know how to describe art, but you can tell which art you like and which art you don't like when you see it. Basically, if you can't put your finger on exactly why you like something, you might say "I know it when I see it!"