To find it definitively, you'll need to:
* Consult your owner's manual: This is the best resource. It should have a diagram showing the location of major components, including the ECM.
* Look for a large, rectangular box with numerous wires connected: The ECM is typically a fairly large electronic module.
* Check common areas near the firewall: Look along the inner fender wells, on the firewall itself, or possibly under the dash (less likely).
If you can't find it in your owner's manual or by visual inspection, searching online for "1993 Chevy Van [engine type] ECM location" (replace "[engine type]" with your van's engine, such as 4.3L V6 or 5.7L V8) might yield results from forums or repair manuals with pictures. A picture search on the same terms may also help.