There is lots of science thrown around about how to gauge whether you are lifting heavy. But "heavy" is a subjective idea; what is heavy for one person may not be heavy for another. it is something we feel, not something we can empirically label. While science can boast and brag about it's findings on heavy-lifting efficacy, in the end gauging the heaviness of a lift is a psychological idea, not a scientific one.
So, we loved when this flow-chart was found recently by Steven from the Spring '14 Team off of www.reddit/r/weightroom. It rates heavy on a scale of 1 to 10, and then gives you a series of binary yes-or-no questions you can ask yourself after lifting to tell if you have truly lifted as heavy as you planned.