The way to check is by two methods. Firstly, look at a sites Alexa ranking. Its an independent method of estimating a sites traffic. The lower the number, the busier the site. Secondly, look at the number of visitors online simultaneously, shown at the bottom of the forum. This forum shows the last 15 minutes of visitors. Another forum I know of (ahem) uses the last 60 minutes, which inflates the number. Go figure.