You could consult dictionaries published at the time the document was written. But dictionaries contain alternate meanings for words, and some dictionaries have meanings that are inconsistent with other dictionaries. How would you know which dictionary to choose and which meaning the authors intended?
You could read other things the authors of the Constitution wrote to try to discern what they meant. But the Constitution was written by a number of people who disagreed on many things, especially on what the Constitution should say and mean.
You could look at how others have interpreted the document. But over the hundreds of years the Constitution has been around, judges and other people have interpreted it lots of contradictory ways.
You could give up on interpreting the Constitution altogether and let some expert tell you what it means. This is a very popular approach, especially with judges who claim to base their decisions on a strict construction of the constitution. The problem is, who do we let decide? Often, judges rely upon people with recognized expertise, like university professors who have researched the Constitution and its history. But the experts often don't agree with each other, and they are as susceptible to error and bias as anyone else. You can find an expert who supports pretty much every possible theory of what the Founding Fathers meant.
People who don't agree with a particular interpretation of the Constitution may point out that times have changed and that the Constitution, which was written before modern technology reshaped our lives, must be made relevant to today's world or it will cease to be useful in guiding our government and our people. On the other hand, people who agree with an interpretation of the Constitution may argue that while technology may have changed, fundamental principles do not. The same people who rely on either of these arguments may use the other argument when it suits them. We see this when people who decry judicial activism in protest of a decision they dislike applaud the same kind of activism when it results in a decision that is to their liking.
Ultimately, it is not only impossible to know what the Constitution was originally supposed to mean, it is irrelevant. What we need to keep in mind is what the Constitution was supposed to do. It was written to be the framework for a nation that would be governed differently from the way many other nations at the time were governed. It was meant to establish a relationship between the government apparatus and the people.
It was not meant to be the law. Instead, it provided a mechanism for making, interpreting, implementing, and enforcing laws. It was not meant to be immutable, so it set out a procedure by which it could be amended. And it was not meant to be the property only of scholars, historians, linguists, judges, or experts. It belongs to all of us, so ultimately we, the people, acting as a nation and not just as individuals, say what it means and what we need it to mean.