Update: Added Rollup 3
For quite a long time Aaron Elder has posted the build numbers for Dynamics CRM. Unfortunately the rollups had not been included in his list yet, though I needed to check on a system which rollup had been installed. You can check that by using the SQL Management Studio and execute the query:
select * from BuildVersion"
This will return the current version of CRM. When you have identified what the build version is, then you can map that to one of the versions here and by that determine which rollup has been installed, if any at all.
These are the build numbers for Dynamics CRM:
Rollup 1: 4.0.7333.1213
Rollup 2: 4.0.7333.1312
Rollup 3: 4.0.7333.1408
If your version does not match any of these, then you're either working with a prerelease version or more likely you have applied a specific hotfix.