Answer: Thanks for the email. Actually your question has two parts. Let's first tackle the empty middle seat question. Based on your writing, I'm assuming you are flying a plane that includes a configuration of 3 seats in a row, and you are hoping for an empty middle one. Aren't we all! =) It's impossible to determine whether you will have an empty middle seat because seat changes can occur at any time, including at the gate. Not until the boarding door closes can you be certain to score an empty seat next to you.
If you are curious about how full the flight is overall, which can sometimes give you an educated guess whether middle seats will be occupied, there are a couple ideas:
First, check the airline seatmaps. Many airlines allow you to pull up your reservation on their website, then modify your seat assignment. Here you can get a general idea of how full the plane might be.
Some cautions though. Some people have confirmed reservations but no seats assigned yet. The airline may block certain seats because they are carrying extra cargo and don't want to fill every seat with passengers (weight restrictions). The airline may also block seats for security reasons, for high-status passengers, or for airport check-in only. All that taken into consideration, it may be tough for an accurate appraisal of how full the flight will be simply by looking at the seatmaps.
Second, check the class availability of your flight. This will give you an idea of how many tickets the airline is still selling (and their respective fare class). Many cheap seats still available could be a sign the flight is not full. Reversely, if you see the flight is nearly sold out, then expect a cramped flight (or perhaps lots of cargo).
In summary, using both the strategies above could help in determining the flight load, but factoring in variables such as last minute flight cancellations, displaced passengers/crew and stand-by customers, a simple guess might be just as accurate.