Anyone from Springfield Mo area? Need help with slant

I'm wondering after talking with my father if the plugs are getting full voltage or not. The Ide comes from the way the spark was while testing. Yes it was blue but wasn't large or anything impressive. So seems like the heat is there but doesn't seem to have enough juice. So them the question is what could cause the voltage to not be sufficient to turn an engine over and how would I go about checking how much voltage is hitting the plugs or is there a way?