What instrument measures inertia?

Answer 1

accelerometer
An Accelerometer.

Acceleration due to gravity can be measured with weighing scales.

Clocks may also be used. At very high ('relativistic) speeds, the apparent speed (rate of change) of a distant clock will change perceptibly (subject to relative vector). The rate of change OF this change will then allow you to calculate your acceleration.

i.e. For an observer moving away from the clock at 0.5c, the emitted signals will be received around half as regularly, so the clock will appear to run slower. If you are accelerating then the apparent 'rate of ticking' of the clock will change in accordance with your acceleration.

Answer 2

In Newtonian physics momentum is measured as the product of the mass and component velocity of a body.

usually measured by R=pc/qB (in Gaussian units).

dont know of any instrument

Answer 3

Acceleration is measured by accelerometers.