I bought a use 1996 Audi A4 2.8l V6 quattro automatic, 132k miles.
I don't know if/when the timing belt service was done.

Is there a way of 'diagnostic' if the belt should be changed?

I don't want to go through the hazzle if it is not needed (since it may have been done recently).
But I don't want to wreck the valves/heads (and more) either.

 

As far as I know, no.

 

What are the 'official' change intervals for the timing belt for the 2.8l 30 valve?

 

I think you will soon find out from the smart knowledgeable individuals on this board, that the best diagnostic to find out if you need to do a t-belt job, is to get a new t-belt.

If you have no record of the last time it was done, just get it done.

 

Yup. Will prepare for the change. Also, the front of the car has to come off anyway (trans lines in the radiator seem to have a leak, which caused water to get into the trans and ruined it). Best time to do the timing belt.

I will go for the belt, tensioner, pulley and water pump (last time, on 1.8t AEB, I got the 'full' kit from blauparts; will read up what else the 'full' includes vs. the 'medium'. Minimum seems to be belt and tensioner IMO).

 

You can inspect the timing belt but that won't tell you anymore about its condition than Ms. Cleo could. Usually, the part that fails isn't the belt, but the tensioner. Tensioner gives up, belt starts floppin around and boom. It may seem wasteful to just change it without knowing when it was done last, but its better than a $3k top end rebuild.

The most widely accepted interval for changing the belt is between 60-90k miles. While you're in there, you should do the cam/crank seals, water pump, t-stat, and serp belt. Blau sells a kit with all that included for like $250, well worth the money IMO