First of all, if you're not a Structural Engineer, it's in your best interest to find a licensed Structural Engineer to do this work for you.
That being said, determining the adequacy of a beam for certain loads (and the maximum load acceptable for certain beams) is directly related to the unbraced length of the member and the member's section properties. The easiest way to determine these load limits is to consult the AISC Steel manual (ASD Part 2 or LRFD Part 4, depending on what code you're using). This reference has tables for maximum allowable (or factored) uniform loads for beams laterally supported and charts for beam design moments. If you know the unbraced length of the beam and how to properly read these charts, then the information you seek is simply attained.
If you're not in the U.S., then you need to reference the corresponding code. I'm only familiar with AISC.
Hope this helps!
Matt