Detecting mobile phone in airplane mode
Broadly speaking, you can't. Any operating piece of electronics will emit EMI, which you might be able to detect with a sufficiently sensitive antennae, but the amplitude is low by design, and likely to be swamped by other sources of EMI in the area. You'll also detect many other electronic devices, like pacemakers and digital watches.

All of which is moot, of course, if the person simply turns their phone off.