One popular tool is the Flesch-Kincaid Readability Score. See, for example, http://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_test. Basically this counts the average number of letters per word and the average number of words per sentence and runs these through a formula to come up with an "appropriate grade level" for the block of text.
Personally I think such scores are interesting but I wouldn't take them too seriously. Yes, long sentences tend to be more complex and long words tend to be less familiar. But the key word here is "tend". The whole point of the FK score is that a writer can deliberately try to reduce the grade level of his writing by replacing long words with short words and breaking up sentences. But it's not at all clear that that really helps.
Consider this sentence, which I just grabbed off the Internet by searching for a fairly obscure scientific concept:
In cosmology, a Hubble volume, or Hubble sphere, is a spherical region of the Universe surrounding an observer beyond which objects recede from that observer at a rate greater than the speed of light due to the expansion of the Universe.
Flesh-Kincaid Score: 22
Okay, so now let me reword the paragraph to reduce my score.
In cosmology, there's a thing called a Hubble volume. It's also called a Hubble sphere. It's a sphere-shaped area around someone looking at space. In that area things recede from the person looking at a rate more than the speed of light. This is due to the expansion of the universe.
Flesh-Kincaid Score: 11
I cut my score in half. Is the paragraph now really more understandable? Did I really do anything to explain the concepts more simply? I don't think so. I just manipulated the scoring system.
My point is: I wouldn't rely too heavily on simplistic scoring systems or guidelines. Rather, you have to apply intelligence.
Using shorter words is a good idea in general. But there are lots of short words that are unfamiliar to most people, and plenty of long words that are well-understood. If you know something about chemistry, try explaining "mole" to a third-grader. I think he'd understand a long word like "wonderful" much more easily. I'm sure most people know what the word "object" means and know what the word "oriented" means, but that doesn't mean they'll easily understand the computer concept of "object-oriented" programming. Etc.
Many suggestions on how to make writing more accessible are rushing through my head, but you didn't ask for a list of ideas, you asked for a reference, so I'll quit here.
And this all reminds me, I heard some guy on the radio once saying that he got new word processing software that had a feature to calculate a reading level for your text, I'm guessing a Flesch-Kincaid or something similar. And, he said, "I have two Masters degrees, but no matter what I do I can't get a score above 9!"