A few month back i bought a 2004 rx8 with 118k miles on it from an auction, what i wanted to know is how do i check if it has the original engine on it?

 

Call a dealership... if they don't have a record of it then it's probably the original unless the owner did a swap themselves...

 

The engine serial number will not match the cars VIN. Look for the little things, like hose clamps that have been replaced, zip ties that are not original or missing, etc.
And yes, like the prevous poster said, Mazda keeps a service record of each car. Any dealer will be able to print this out for you if you ask nice.

 

The remans also have a tag on them, there are pictures posted up here showing what to look for. Search.

 

I'll say 95% its a reman, specially if its an 04.