How can you differentiate between a contact force and a force at a distance?

To differentiate between these two forces we must first understand the meaning of them:-

Contact forces- Those forces in which there is a physical contact between the surfaces on which force is applied.

Noncontact forces-Those forces in which there is no physical contact between the surfaces and the force acts between them from a distance on which force is applied.

Hence to differentiate between these two forces we must look for physical contact between the surfaces. If there is physical contact then it is a contact force otherwise it is non-contact force.