It's time to get one.
If you are working with electricity, you should 100% have a DMM and know how to use it. For your own and other's safety. Especially if you are working with 120V wall power. It's critical to know what you are doing. This is not an option. Plus, you need one to check the 5V and other voltages that the game board needs, to make sure they are accurate. Otherwise you can damage the hardware inside your games.
Get one, and youtube some tutorials for how to use it. The UNI-T UT58D or UT58E are decent affordable DMM's. They go for around 50-60 bucks on ebay or Amazon.