How can I access redirected drives that are done via the RDP drive redirection options?

These drives typically show up in the windows gui as C on <your - client - name> , but to reference them for working directories or files you can use : 
\tsclientcdocumentsmydocument.doc 

If you want to copy a file from terminal server c: drive to local workstation d drive : xcopy C: empmyfile.txt \tsclientDdata 
If you are connecting from a Macintosh client, the syntax is: xcopy c: empmyfile.txt \tsclient<Your-Mac-HD-Name>data 

You may also use these as the working directory when launching a published app. Not all apps support the working directory and this also does not limit them to just that directory but rather as a default starting point for those apps that support it. 

If you need to be able to see how much free space your local redirected drive has you can trick windows into showing this by mapping the drive using the net use command. 
To map drive R to your local drive C you can use : net use r: \tsclientc then view the properties of R: