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

I'm just realizing you probably meant web crippling due to concentrated loads (such as at bearing plates). Geez. This'll probably help you a lot more, if you still need it:

At the end of a beam, use: R/[(N + 2.5k)*tw] <= 0.66Fy
At an interior concentrated load, use: P/[(N + 5k)*tw] <= 0.66Fy

Where: N is the length of the bearing on the web;
       k & tw are given in AISC manual;
       P & R are the load and reaction on the bearing area, respectively.