I started the campaign against worn out fuel pumps, and Howie's posted the source for the cure.
To answer your questions about symptoms (and they will probably vary) - The only clue I had was when the bike vapor locked and quit. But, there were symptoms that went unrecognized until the fix - the bike ran strong and would go to top end. But, it surged and bucked at off-idle "parking lot" speeds and didn't like lugging below about 68-70 mph in 6th gear (with stock sprockets). These are accepted as normal behavior by a lot of Ducatisti - part of the bike's personality.
After overhauling the pump, I immediately noticed improved low speed driveability and suddenly the bike would run as slow as 60 mph in 6th gear without complaining or that lugging feeling.
My theory is that since the pentagon pump is rated for about twice the fuel flow the 900 requires at max hp, a dying pump can still flow enough. But what probably happens is that the delivery pressure is falling or erratic as the diaphrams get stretched and hardened. High end carbs like Webers and Mikuni CVs are sensitive to delivery pressure at the float bowl and it can affect metering accuracy.
This was reinforced in my case by spark plug readings. Before the pump fix, around town riding resulted in carbon fouling and highway trips results in clean normal plugs - I blamed it on the jetting and lived with it. After fixing the pump, the plugs are textbook perfect no matter what kind of riding I've done.
After checking for pinched hoses and clogged vents, the fuel pump is the next easiest and cheapest thing to eliminate from the suspect list before getting into the carbs. If that doesn't fix it, then you know the system is good up to the carbs, and you can tear into them.