this doesnt "directly" have to do with skylines but.....

...how exactly can you prove that there is a subsatiantaly similar version of a car here in the US as there is in Japan. I see 1992 SiR's and 180SX's alot now, so I was wondering how is this done?

The cars would have to be just about identical. I'm not just talking about looks. Crash/bumper structures would have to be the same, glass thickness, etc.

You'll have to do research on those things to see how the US and JP spec cars match up.

This is directly from NHTSA's iport guidelines:
While there is no specific restriction on importing a right-hand drive vehicle, an eligibility number based on a substantially similar U.S.- certified motor vehicle may not apply. Our experience has shown that the safety performance of right-hand drive vehicles is not necessarily the same as that of apparently similar left-hand drive vehicles offered for sale in this country. However, we will consider them "substantially similar" if the manufacturer advises us that the right-hand drive vehicle would perform the same as the certified left-hand drive vehicle in crash tests. If the vehicle is not substantially similar to one sold in the U.S., the RI would have to demonstrate that the vehicle, when modified, would comply with the applicable Federal motor vehicle safety standards, which could involve crash testing several vehicles.

So, either you get a letter from Nissan or Honda, etc. Which they won't risk doing because if you get into an accident, it leaves them wide open for a lawsuit. So, that leaves going through a registered importer.

Here's the link to the info I quoted, which is on the vehicle eligibility list.
http://www.nhtsa.dot.gov/cars/rules/import/ELIG102803.html

the only car I found on the list that would be considered for importing over here is the 1988 240SX (??) which is wierd because the US (and CDN) is the only country I know of that got the "240"SX. Everyone else got a 200SX and the 180SX..