From the command line, you can run the command "du". This is disk usage reporting. It has many options (which you can get the full detail of with "man du" at your command prompt). The ones I normally use are -h - for human, and -s for summarise.
The dot here specifies the current directory, but could be replaced wiht any path.
It may take a while depending on how many files are in the directory.
If you are looking for graphical means, most file managers will probably allow you to right click on a folder and get the disk space usage.