“Right Hand Rule”
If the force on the beam creates a positive reaction, you can apply the “right hand rule” in which case the reaction about a point creates a clockwise motion about a certain point.
An easier way of determining the sign of the bending moment at any section is that upward forces always cause positive bending moments regardless of whether they act to the left or to the right of the exploratory section. – See more at: http://www.mathalino.com/reviewer/mechanics-and-strength-of-materials/relation-between-load-shear-and-moment#sthash.U1w2fNUF.dpuf
[source: http://www.mathalino.com/reviewer/mechanics-and-strength-of-materials/relation-between-load-shear-and-moment]
Probably the best video I’ve seen on explaining how to determine positive and negative moment