Question

How can we tell when forces are acting on an object (science)

Answer 1

If an object's speed changes, or if it changes the direction it's moving in,
then there must be forces acting on it. There is no other way for any of
these things to happen.

Once in a while, there may be a group of forces (two or more) acting on
an object, and the group of forces may turn out to be "balanced".  When
that happens, the object's speed will remain constant, and ... if the speed
is not zero ... it will continue moving in a straight line.  In that case, it's not
possible to tell by looking at it whether there are any forces acting on it.