instrument to find mass?

Answer 1

A scale would usually do.
The best is when you comparing a test mass against a standard mass.

Answer 2

Besides scales that use weight to judge mass, there are also devices to measure mass while weightless, in space.
An inertial balance will judge mass by how much force it takes to move something back and forth, since more mass always means more inertia.

Answer 3

Mass can only be compared.A double pan balance (Common Balance) is usaully used to find the mass of a body'.