It's easy to tell if you disassemble the tweeter. The coil will be crusted over with it and need to be cleaned.
FerroFluid aging
I know the topic has been discussed previously but this one is a tad more specific.
Did B&W use ferrofluid in the 800 matrix (801 - 805's) series (1-3) tweeters?
If they did, is there any known issues with the drying out of the ferrofluid in the 800 series specifically? Yes I had some Vifa tweets that it happened to, but sadly my 805's died from other causes so I don't know if they would have lasted.
Any info would be appreciated.