You have a few options here. Simplest/best I'd say: do an Open... on a "file:" comm handle. (Perhaps followed by a GetNumCharsWaiting to get the file size, in case a 0 for that means "doesn't really exist" for you.)
Fancier method if you're using lots of different files and need a listing, would be to use an "ftp:" comm handle, specifically the "dir" option with SendCommunicationHandleCommand.
BTW, also check out the GetAvailableFileSpace commandwhich you can use to keep track of how much file space you have remaining in your on-board RAM, flash, or microSD card. Happy "filing"!