I did nothing to prepare for it. I used to get a cel occasionally because I removed my egr pipe, but I hadn't for about 1000 miles, so I was feeling lucky.
Home in the Milwaukee area, they only plug into the OBD port for '96+ cars, but can put it on the dyno and do the sniffer test if they choose to.
I drive up to the guy at the station, he makes me get out so he can plug into the car under the dash. When he's done, he comes over to talk to me and says,
"That's a nice looking car, I was gonna warm up the dyno and take it for a spin, but I saw the intercooler and thought I better not, just in case."
It must have been my lucky day, especially since modifying the stock intake plumbing in any way is illegal here.
Long story short: I passed OBD II emissions testing with my turbo and no egr pipe.