How do I check the timing on a [DSM]?

You would already know this if you had checked your shop manual. (You do have a shop manual, right?) Also, owners of late 1996 to 1999 DSMs need read no further - base timing cannot be adjusted on these engines - any attempt to do so will result in a 'check engine' light.

Adjustment of the base engine timing is done through adjusting the crankshaft angle sensor. There is a trick, and here it is : the ECU normally controls the engine timing, so unless you disable the ECU control, you are not seeing the base engine timing when you hook up your timing light. Instead, you are viewing the ECU-controlled timing, which may be substantially different. Adjusting the timing without first disabling the ECU control will have little effect, as the ECU will re-adjust the timing to the original value.

Fortunately, the factory provided an easy way to disable the ECU timing control - a small electrical connector in the engine bay. All you have to do is connect it to a ground to temporarily disable ECU timing. On 1Gs and 1995-1996 2Gs, the connector is located on the firewall, near the middle. (Owners of 1997+ cars please note: these engines lack the timing adjustment connector, and timing cannot be adjusted.) 

After you ground this connector, hook up the timing light and check if the engine timing is +5 degrees BTDC . (For instructions on how to use a timing light, check Brad Bauer's Timing Light FAQ.) Timing adjustment is done by adjusting the crankshaft angle sensor. After the adjustment is complete, unhook the connector and re-check the timing, which (owing to the ECUs intervention) should now be around +8 degrees BTDC.

Please note that it is possible for the operator to set the timing further ahead than +5 deg BTDC. Doing so will not generally help power output, and may limit or disable the ECUs ability to safeguard your engine. See Chapter 6 in the ECU Primer for details why.