There is not one specific answer to this question. The website efunda.com offers a simple calculator that can give an estimate as to what the capacity of the beam is based on the properties that are input.
For an easy online beam capacity calculator, go to efunda.com and click on I-beams under the Design Center section. From there, follow the instructions and input the loading data.
A licensed structural engineer should be consulted when determining the capacity of a beam for structural applications. While the calculator mentioned above is a good resource to give an idea of the beam's capacity, it may not take into account all applicable building codes and standards. A structural engineer can properly evaluate the beam based on its orientation (as a beam or column), size, steel grade, length, bracing and the types of loading that the beam would experience (live load, dead load, snow, wind and seismic).