If you were expecting to come here and find a great troubleshooting guide for determining conclusively if the DME computer is bad, I apologize. There's actually no good way (currently) to test the DME computer. Perhaps someone, down the road, will come up with a good method for bench testing the computer. Until then, the only test you can really do is to try the suspect DME computer in another car OR borrow a know good DME computer to test in place of the suspect computer.
The good news is that the majority of DME computer problems in 944s are related to bad solder joints which can be repaired. If you are having problems starting the car and suspect a bad DME computer, try tapping on the computer during cranking. If the engine fires while tapping on the box, you have bad solder joints. If it still doesn't start, that doesn't mean the DME computer is good. It may be good. But, it may also mean that the solder joints are very bad or the DME computer has failed for another reason.
The remainder of DME problems are primarily related to water intrusion, especially the later 944s where the DME computer is located in the passenger's footwell below the battery tray.
Remove the DME computer from the vehicle using DME-01.
Using the appropriate steps of DME-02, open the DME computer as if you were going to replace the computer chip. The DME computer has two separate circuit boards which are mounted facing each other and are connected by a flexible ribbon cable. In order to inspect the circuit boards, you'll have to separate the circuit boards. This process is explained in DME-02.
When you have the two circuit boards separated (still connected to each other by the ribbon cable), inspect all the solder joints with a magnifying glass. If the solders are dull grey in appearance as opposed to shiny silver, they should be resoldered. Most solders now have some type of rosin core flux to promote better solder flow. However, the flux will frequently leave a brownish residue on the solder joint which many manufacturers neglect to clean off. This can make a good solder joint appear bad. So, before you condemn what appears to be a bad solder joint, first clean the joint with a cotton swab and alcohol to remove any flux residue.
When you inspect the solder joints on the circuit boards, pay particular attention to those joints at the connectors for the ribbon cable that connects the two circuit boards. Historically, those solder joints are the most susceptable to being broken or otherwise bad.
Then you inspect the inside of the DME computer, look for evidence of water intrusion. Typically, this will be a white residue on portions of the circuit boards or as a visible white line (water line) on the inside of the computer casing.
After you've inspected the DME computer, repair any questionable solder joints.
After you've repaired any questionable solder joints, reassemble the computer, install, and attempt to test drive the vehicle (assuming the DME is now working), to check the operation of the computer.