This is question actually has two answers. The first answer is that since a force is a push or a pull, you just look for all the things that are pushing and pulling on your object. At first this seems easy, but remember you may not be able to see all the things that are pushing or pulling on your object (note it took many people a lot of hard work to figure out in a mathematical way that the earth pulls on us). Beyond this method we have another resource. Remember that Newton's first law tells us how objects behave when they feel no net force, so if we see an object behaving in a different way (i.e. not uniform motion) then we conclude that it feels a force. The caveat is that Newton's first law concerns the net force, so the object may be moving uniformly because all the applied forces it feels cancel. In this case we are left back with only our imaginations and powers of observation to try to figure out all the forces on the object. In these exercises there are only one or two forces acting on an object and so hopefully it won't be too hard to find them all.
© 2008 Kansas State University Physics Education Research Group.