I'd like to check the patch level of my Nexus. I'm using Lollipop 5.1.1 build LMY48I. Under 'System updates' in 'About phone', it says 'Your system is up to date'. Should I assume that includes security updates?
In Marshmallow (Android 6.0) there will be an additional bit of information on the 'About' screen showing the "Android security patch level". This should show the date of the last security update that your phone has. How you will know that this is the latest available is unclear though.
As for pre-Marshmallow the best suggestion I have is to look at the factory images page and see what the latest build number is for your device. Depending on which device you have there should be a newer version than LMY48I, most likely build LMY48M.