A 3.4 will have a raised boss on the "trunk" side of the block, for the crank sensor. It will have a threaded hole, for a bolt, and a small metal plug, where the sensor would fit in. (88 2.8s also have this boss, but all it has is the threaded hole for the bolt. The larger sensor hole is not drilled.)
It will also have a plug in the top surface of the block, just above the timing cover, where the cam sensor would go.
A 3.1 might also have these plugs, but I'm not sure.
As posted before, the large "F" cast in the side of the block will be a dead giveaway.
If you can see the block where the starter is bolted...
On a 2.8, the ear that sticks out will be flush to the top of the starter, the entire width of the ear.
The bottom edge of a 3.4's "ear" will be beveled at an angle, leaving a bit of a gap towards the outside end.
[This message has been edited by Raydar (edited 03-04-2015).]