Turbo Boost - How do I find out how much

If its a GU 7/8psi.

only way is a boost gauge... Installed one in my GQ the other day, took 30 mins. ALso fitted EGT and proper Temp Gauge.

A VDO boost gauge, 0-15 psi cost me about $80 yesterday, easy to fit. Nice to know what it's doing. Mine runs 11psi, my son's runs 12 which kicks in fully at about 2000rpm onwards (from there it's limited by the wastegate.)
Less than 2000 it's less boost, eg: about 2psi at 1100 rpm.

Agree with all above responses Zapper.

Depending on who you go to, what type of turbo, what type of setup etc will vary psi, power and when it cuts in, mine has been setup to cut in at 1500 rpm and depending on ambient temp, psi varies. If my thermo fan is in, and I'm pushing 450-550 degrees, my psi is down as the turbo obviously runs more efficiently when everything is cooler.

If it is a GU, Truckster is right, around 7.5 I was told by Nissan I think 7 or 8 would be more accurate. And another comment was right on the power of the GU TDI with the turbo at 8psi, got into the GU out of mine and thought I was driving a naturally aspirated. Apparently 11 or 12psi gets the GU's up and at em' without putting the mechanicals at any risk, but your warranty will be stuffed!

4 degrees today, my thing nearly took off going up the hill that it usually hates!!! AWESOME, im moving to Tassie where its always cold