On most devices, the SDCard is either mounted or at least linked to either
/sdcard. However, when the device features both, an internal and external SDCard, the latter usually is to be found "inside" the former, e.g. at
/sdcard/external_sd. Alternatively, it may be mounted at
/storage/extSdCard. Note the capitals. Android filenames are case-sensitive.
For a user, the easiest way to figure out where it is (or in your case, rather where your ''file'' is located), is using a file manager which allows you to not only browse your SDCard, but the entire system. I for example use ES File Explorer: If I start the app, it automatically places me on my sdcard (I didn't configure it such, so I assume that is as it always works). The path is displayed at the top of the display, so you'd see it immediately. If it's too long to be displayed there (even if you switch to landscape), you still can "navigate up" until you only see
/ there, and note all steps – so you would end up with the complete path.