Cut the member at the section of interest
Isolate one side of the cut as a free-body diagram
Assume the internal force directions using a consistent sign convention
Axial force: assume tension pulls away from the cut
Shear force: assume a direction consistent with the chosen sign convention
Bending moment: assume a direction consistent with the chosen sign convention
Torsion: assume a twisting direction consistent with the chosen sign convention
Apply equilibrium equations to the isolated section
If the calculated value is positive, the assumed direction is correct
If the calculated value is negative, the actual direction is opposite to the assumed direction