Consider the ends of the beam--the loading is supported by the shear perpindicular to the length of the beam. The load is the same as used for the beam in bending, it is just concentrated at the support. The shear is the loading divided by the beam cross section--for instance a 2 by 4 has a cross section of 8, a 3 by 4 has a cross section of 16.
Now take the load at the support and divide by the cross section--that's the shear of the beam--compare this shear to the allowable shear of the beam material (usually established by the vendor). If the shear is greater than listed for the material with a safety factor (multiplyer, usually more than three) its good, if not use a beam with a bigger shear strength.
BTW a good structural engineer or an experienced contractor effectively does this for trusses, rafters, and joists--although a good fule of thumb (within reason--eg normal live floor loads 50#/ft^2 ) for pine joists supported at the ends (nominal 2 inch width on 16 inch centers) is 1 inch of depth for every 2 feet of span plus 2 inches. That is 2x4 joists on 16" centers is good for 4 foot spans, 2X6 good for 8 foot, 2X8 good for 12 foot, 2X10 good for 16 foot.
Caveat--cantilever joists and beams with cantilevers over 2 foot should be individually engineered.