You can't. You can only measure acceleration (with an accelerometer). If you get zero acceleration, you're in an inertial frame of reference and for all intents

You can't. You can only measure acceleration (with an accelerometer). If you get zero acceleration, you're in an inertial frame of reference and for all intents and purposes in zero gravity. Newtonian physics says you're technically wrong, but general relativity says Newton is wrong so you're okay.