The only correct answer is to use a version comparison function.
A lot of the answers here make really poor assumptions that even a little critical thinking can show to be incorrect, like assuming that each sub-version is one digit, or that jQuery will never reach version 2. I do not want to re-hash them in this answer, but those periods have meaning: It means “whatever is on my left is a number that matters more than everything to my right.” And therefore the correct solution always checks the leftmost version number component, and only checks the next component if the first pair matched exactly, and so on.
jQuery’s version number is available in $.fn.jquery
as noted in many other answers here. It is a string, and your version comparison function should expect strings.
Here is a port of PHP’s version comparison function to JS. Note it’s probably overkill for jQuery, especially if you assume you’ll only ever see numbers and won’t be dealing with release candidates and the like. But it’s going to be reliable and future-proof.