Since [ is a meta-character in wildcard expressions, you'll need to wrap it with another set of brackets:
SELECT PATINDEX('%[[]%',mytextfield)
Lutz
A pessimist is an optimist with experience. How to get fast answers to your question[/url]
How to post performance related questions[/url]
Links for Tally Table [/url] , Cross Tabs [/url] and Dynamic Cross Tabs [/url], Delimited Split Function[/url]