Chester needs to get the credits for the findings in the following blog as he pointed me to this specific behaviour!
MCMS does not have a documented method to identify if the current user is a MCMS administrator or not. But there is a very easy way around this if you are using the CmsHttpContext!
The CmsHttpContext has a UserCacheKey property which is used by ASP.NET output caching with MCMS in combination with the VaryByCustom="cmsrole" method to ensure that content can be varied for different user roles.
The MCMS help gives the following statement for this property:
UserCacheKey is guaranteed to be the same for two users, of those two users are both members of all the same MCMS rights groups. UserCacheKey will also be the same for two users if both users are MCMS administrators. In contrast, UserCacheKey is different for two users if those two users do not meet either of the above two requirements.
Looking at the value returned for this property for MCMS administrators shows that always "A" is returned!
So a simple check for this property for the string "A" is sufficient to identify if the current user is a MCMS administrator or not!
Actually there is no guarantee that this property value will be the same with the next service pack. But the chance that this gets changed is very low.