There are various types of 'sludge'.
On a BMW, the most prevalent kinds of sludge a motor will get when properly maintained, ie. oil changes LESS than the 15k mile interval will be the thin coating on the various parts.
Notice the brown coating on my Vanos assembly. Stuff like that..
The stuff that you really have to worry about is the crap that lays in the bottom of your oil pan. That is the stuff that can restrict oil flow, clog oil passages etc.
When you use a harsh quick type of flush that you pour in with or in place of your oil, you do run the risk of breaking up the heavy sludge and restricting oil flow.
Because BMW's are supposed to use real synthetic oil, not a synthetic blend, real synthetic oil has heavy cleaning properties and the the build up of the heavy stuff (sludge) is greatly reduced. To answer your question, I guess you could such the oil out of the bottom of the oil pan with a oil evacuation pump or something similar and look for the heavy stuff. Or if you had access to some type of Bore Scope, you could look inside your valve train that way. It is really easy to pull your valve cover off......
Slow dissolving cleaners (like AutoRX) slowly over time, clean the engine and allow the oil filter to trap the contaminants.
Since I bought my car with 47K on it, and it was four years old, I do not know how it was maintained, but as you can see, I bet it was at the 15K intervals. After doing the Vanos Oil Seal repair, I used AutoRX and after 8K miles (two treatments), I took the top off the engine again. I did not take a picture but all that brown crap was gone and everything was nice an shiny. If you halve the oil change interval down to about 7-8K miles, which is what I do now, then the synthetic oils natural cleaning properties will greatly reduce or even keep that brown crap from building up.
I have seen this on other Z4 forums.
Thanks
