Lets see a show of hands. The weather is getting warm now.

If it's bad, The temp gauge will certainly tell ya, If your method works for fc's on there way out, That's awesome, I've only seen them working or not, I have'nt received much warning before other than howling, That go's for these things and my cat engines.

Replaced mine last week, although I did the traditional rolled up newspaper test. Now that I have a new one again, it's hard to understand how I missed the fan roar. I guess it dies gradually so you notice less and less... but still! :confused

Replaced mine last month, sometimes i think the new one is broken as i don't always hear the roar when the engine is hot. I think they have a mind of their own.

It could be defective. Your fan should be roaring when hot.
See this post
http://www.m5board.com/vbulletin/e28-m5-discussion/99751-attention-fan-clutch-do-not-buy-behr-buy-sachs.html

Engine hot. Have someone shut off car and observe fan blades if they spin more than 3 times you have a defect.