I am trying to determine if my main relay switch is bad in my 1990 Honda Accord LX. Is there a test for it before I purchase one?

If your starting problem is not temperature-related, the main relay is not likely the culprit. Having said that, there are bench tests that can be run on main relays, however I place little faith in them. It's better to replace them every 120,000 miles or 10 years, whichever comes first.

In most cases, what appears to be a main relay problem (effect) is really caused by ECM failure. Perform the K-Test on the ECM, before condeming the main relay.