Final answer:
Forces are not in equilibrium if an object is slowing down, changing direction, or speeding up, as these indicate the presence of a net external force causing acceleration.
Step-by-step explanation:
Observations that indicate forces acting on an object are not in equilibrium include situations where the object is slowing down, changing direction, or speeding up. These are all forms of acceleration, which occur when the net force on an object is not zero. Objects can also have a non-zero velocity in a situation without external forces acting on them, such as a puck gliding on smooth ice after the initial force stops acting on it; it maintains a constant velocity due to the absence of friction.
When forces are not in equilibrium, the object's rate of change of momentum is not constant, and so, if a constant, nonzero force is applied to an object, we can expect the object's velocity and acceleration to change over time, not remain constant.
By choosing different systems of interest, you can change which forces are considered relevant when applying Newton's second law of motion. An object moving in a circle at a constant speed may not have a net external force acting on it in the direction of motion, but there is a centripetal force enabling the change in the direction of the velocity.