Differentiating audibly between bad caps and bad Ferro fluid

Well, if your speakers are old and have FF go ahead and change it.

If you have electrolytic caps that are > 10 years old replace them with modern electrolytic caps. They are inexpensive.

@sifter, take a look at your crossovers.  If there is any indication of compromised electrolytic caps, change them.  Also look for any indication of excessive heat.  I had some old speakers that needed new ferrofluid, and this caused enough heat that the inductor frame warped.  Richard So told me that was a classic sign of bad ferrofluid. 

There comes a point with these old classics where you have to rebuild or move on.  

Agree with @brownsfan address the crossovers first and look for compromised electrolytics, or if they look really old replace them anyway (nichicon non polars and mundorfs from parts connexion are OK). If you decide to use films as replacements get some electrolytics as well, because you not actually prefer the final result.

If that doesn’t fix your problem progress onto the ferrofluid.

Great advice, thank you.

Be careful replacing electrolytics that go to ground through a resistor as the ESR is critically important in those components.  In series with the drivers is less critical.

Old ferrofluid makes the highs sound flat, you lose the upper midrange. I thought it was the best thing when I discovered it after frying my tweeters but after a while, I found out how bad that stuff actually is. Much greater improvement than changing capacitors for me. I'd do both but ditch those tweeters or at least change the fluid first.