Oh they know..!!
what mods that you can't see do you think there might be?
If your profile is correct and you have a non turbo R33 GTS, then it's almost guaranteed that you don't have any internal work.
and everything else can be seen.
if you have a pod filter, it stick out like a sore thumb.
if you have extractors, it will be obvious as they hang off the side of the engine where there would normally be a cast iron factory item.
look under the car, is the exhaust larger than the size of a banana, then you've got an aftermarket exhaust.
you'd be better off going on a cruise with the QLD group and asking someone there to tell you as they can just have a looks for the obvious.
Any workshop will charge you by the hour to tell you the obvious.
and like I said, it won't have internal work if it's an NA skyline so pulling it apart would be a waste of time.