How do I determine the net force of an object when the force on one side of the object is unknown.

If the force pushing against this the object on the left side is 50N, and the force pushing againsts the right side which is unknown(f2)
Then how do I get fnet?

f net simply equals 50 N - f2 in the direction left to right. You can't get a numerical answer without knowing the value of f2, but in physics, there is no reason not to use an algebraic answer.

To add to what elscarta said -- you may be able to determine f_net IF you have some other piece of information, such as the acceleration of the object (because then f_net = ma), or if the object is known to be in equilibrium then f_net = 0.