If you have access to VAG COM, scan for codes, and you should see a low boost code.
If you don't have access to VAG, then to physically check it could be a bit fidly. I changed mine when I did my cambelt, so was very easy to change. You can do it without taking off the front end, I think most people do it by taking off the undertray and accessing it from underneath.
The DV is located below the turbo, and is held in by 3 bolts. Undo 3 bolts, unclip harness, plug in new DV, plug in harness, and screw 3 bolts back on, very easy.
To see if your DV is blown, and if you have the older revision B membrane style, just pull up on the knob around the membrane. With mine there was lots of holes in the membrane. If you've got the metal impeler style one, it shouldn't go wrong.
It is well worth changing the DV to the revision D one, if your car has a blown B revision, you will think your cars had a remap when you put in the D!