Saved-to-disk csv files don't have any metadata storage, so you don't have a bullet-proof way of determining if a csv file is delimited by a comma, a semicolon, a pipe, the word "something", or any other character.
There is a standard for how csv files are created, but, uh, well, you know how that goes...
Microsoft decides to do their own thing, changing the character based on "Regional and Language Options". If your program is running on the same box as the Excel app that made the file, you might be able to programmatically determine that.
I'd say your best bet to infer the separation character is by looking at the first line - the headers. HOPEFULLY you don't have a comma in your header fields, and you can have a whitelist of expected separation characters. When you look at that header line, if you see a comma, you know the whole file is separated by commas, or semicolons if you see a semicolon.
This, of course, is not perfect. If you have any control over making Excel consistently printing csv's with the same separator character, that's always going to be a much better solution. Or, when importing a file, ask the user which format the file is in (comma csv vs semicolon csv, etc).