TRAILS (Tools for Real Time Assessment of Information Literacy Skills) announced updated versions of their grades 6 and 9 assessments, and brand new assessments for grades 3 and 12.
TRAILS offers both general and category specific assessments, such as topic development, identifying sources, etc. which are good tools for both pre and post instruction data-gathering to demonstrate the efficacy of your library program.
Another way to assess skills (and I completely stole this from librarian extroardinaire, Michelle Luhtala) is to create your own assessment using Google Forms. This has some distinct advantages:
1) You can personalize the test to your curriculum/students.
2) The answers feed to a sortable spreadsheet, for easy analysis.
3) If you add a "Do you feel confident, or would you like more help?" question, it is then easy to identify and help students who need further guidance.
I confess, I have not done much (read: anything) with assessment before this, but I plan to start this year.
I am about to start working with 10 and 11th graders as they prepare for their individual IB projects/extended essays; I'm going to create a pre-assessment quiz for both, and I'll post it here when I'm done.