What wattage/cm^2? Microwatts? Attowatts? All microwave seals leak. They may act more like a -60dB attenuator, rather than a perfect reflector.
If you're only worried about microwave heating (burns,) then any straight fluorescent tube-lamp will light up when held against a watts-scale door leak. Or, hold the terminals of an NE-2 neon lamp with fingers, and pass the glass lamp over the door seals, watching for orange flash. This is the same as tesla-coil lighting of a fluorescent tube. (Heh, actually a microwave oven chamber is a driven high-Q resonator. So in that sense the oven IS a 2.5GHz tesla coil.)
If instead you're looking for a high-milliwatts leak which might overload a receiver front-end, or perhaps hurt some sensitive electronics, then Tony_Stewart's trick with the red LED works fine. Simpler: two red LEDs soldered back to back in parallel, so both will light from AC. Add a 2" floating wire to to one side, hold the other with fingers as a ground, then pass the wire near the seal and watch for LEDs to glow.
I once took apart an active oven-leakage detector having a separate probe. The probe itself was a tiny pcb with a 1" ring trace, with six 1N4148 diodes soldered in a radial array, positive terminal pointing inwards to a central contact. The ring was ground, the pos terminal led to a resistor load and a 741 op amp. Obviously it was acting as a small loop, a nearfield probe-antenna with a diode detector. But they'd added five more in parallel, so the device would read roughly the same e-field regardless of probe orientation.