This has been covered in a post, looong ago, but here's a recap
If the airplane is in ground mode, here are a few obvious hints the Mad Dog will throw at you. Given their obviousness it's quite an effective system still for it's enginuity in verbally telling a pilot why the plane is not set for takeoff as is the checklist designed to filter out the chance of the crew not knowing.
if the plane is airborne......
1. You will get RAT probe heating on the ground. this is confirmed in the before starting checks as you turn on the anti ice, and set the knob to RAT current to verify it is at 0.
2. When taxiing out and doing flight control checks the "SPOILER DEPLOYED" caution does not illumintate as it should. This should raise a flag immediately, as the flight control check is vital to any crew.
3. The flight mode annunciators would read GO RND instead of TAK OFF when the TOGA buttons are pressed. So if you have you AP / FD system setup correctly, TAK OFF will not appear in the FMA, but GO RND. A definite clue that the plane thinks it's in the air.
These are the obvious ones. Because iether they are part of a checklist that must be accomplished or because the evidence is in the form of green LED's staring you right in the face with wrong info. Info which you verify verbally before every takeoff.
Indeed, if the plane is in airborne mode, the TO warning system is deactivated. This requires no explanation me thinks.
Xander