How can you tell if an object is in equilibrium?

Definition: An object is in equilibrium if the acceleration of its centre of mass is zero in all directions and its angular acceleration is zero.

In simpler terms:

If the object is not accelerating, it must either be at rest or moving with a constant velocity, as F = ma (force = mass x acceleration). If there is no acceleration then there will not be a 'resultant force' acting on the object.

In a linear sense, a zero resultant force means that the forces are balance in all directions.

In a rotational sense, it means that the total torque about any axis is zero.