Being able to explain it in 5 minutes doesn't necessarily mean it's not complex. It really depends on how elementary the explanation is. In other words, how complex are the terms themselves in which you explained it?

For instance, you might say that the Fourier Transform is just a decomposition of a function of time into complex exponentials representing frequencies. But how does a complex exponential represent frequencies? Can you explain Euler's formula?

I usually pick someone I know well and consider what it would take to make them understand, given what I expect them to know already. I find this really helpful in testing my own understanding, but it's a good way of testing how complex an idea really is, relative to the average person's knowledge. (Incidentally I'd rank Fourier stuffs as pretty complicated, all things considered).