I was just wondering if there are any telltale signs that an engine has been rebuilt?

 

My 87 has a rebuilt motor. The motor came from a Mazda dealer. It has a little circle with an X in it stamped onto the top of the housing behind the crank angle sensor. The motor was clean and the person I bought the car from had an invoice from the shop that installed it. Their invoice had the Mazda part number on it.
Alot of 'real' rebuilders put a stamp or mark on the motors, but I don't know if this holds true for rotarys.