From your question I can't tell if you want to determine the delimiter from an existing csv so you can parse it correctly or simply allow the user to choose a delimiter and do a search replace across the file.

As far as determining the delimiter in an existing file you may want to prompt the user to select a delimiter. Giving them your best guess as well as showing them the first few lines of the file would probably be a good idea. You might even go so far as to show them where the fields would break based on their currently selected delimiter.

Replacing the delimiter could be tricky since delimiters are often chosen based avoiding characters that are likely to appear in the data.