look on the block right where the starter sits.  I don't know the coding structure though.

The E7TE isn't a date; its the model of the heads.  These heads were introduced in '87, hence the "E7" prefix.  The "TE" is an indicator of the original application for the heads, which was the truck line (were it the Mustang it would be "ZZ").  These were the best of the factory heads offered on the 5.0 until the GT40s came out in '93 on the Cobra and Lightning.  The casting number on the heads didn't change until the design of the heads changed, so the E7TEs were used from '87-'93 on all HO 5.0s, possibly into '94-'95 as well, and maybe beyond in trucks until the GT40s took over.

So all I can say is that the heads are correct for Mustang 5.0.  Whether or not its a correct '89 engine for an '89 Mustang is harder to pin down.  The block should have something stamped on a machined pad near the oil pan that can tell you I believe.  The heads, not sure, but there should be something stamped or cast into it.  Look for something that looks like a clock. 
 

The rear pad will have partial VIN serial number stamped in case you’re wondering if engine is original to the car.  Partial VIN is also stamped into my T5 case as well.