Usually the quickest way is to go to something, anything, that they've posted and click on their username at the left, as in the attached picture.

You could also go to their profile and click on the Stats tab, or go to Advanced Search and enter their username. Any of these ways will give you a maximum of 200 results. Some of the advantages of Advanced Search are that you can specify a particular forum to search in, or a particular time frame or sort order, and see posts that aren't in the most recent 200.