Sun might put out some good technology but whatever group is in charge of naming and numbering their release seems to be a bunch of idiots. I don't know about Weblogic, but I am sure somewhere there is a spec sheet that says what JSP version it supports. Looking at the DTD used by your web.xml should give you a hint about the Servlet version you are using. Everybody know that "web-app_2_3.dtd" means Servlet Specification 2.3, right? Based on the Servlet version you should be able to find out what version of JSP is supported.
I think there is a one-to-on mapping between Servlet and JSP versions, but I could be wrong. My understanding is this: Servlet 2.3 > JSP 1.2 Servlet 2.4 > JSP 2.0