Swedish Steve:
Stop suspecting because you suspect wrong.
The B777 uses a strain gauge in the undercarriage support beam to detect weight on wheels.
It seemed a natural, as that's the way the 767 does it, but I do indeed suspect wrong; - thank you for the correction.
HarryMann:
Further to your question regarding cockpit indications, the 767-300 does indeed present an "EICAS" message to indicate a fault with the air-ground sensing system. The message is "
AIR/GND SYS" or "
NOSE A/G SYS". The AOM states that if the message is present, "
Affected equipment and systems will not operate normally and therefore takeoff is not allowed".