From a look at the
ntpd man page this doesn't look possible. It does save out stats of what it is receiving but it will always try to set the time. You could in theory hack out those system calls but that sounds dangerous/over the top.
If you just want to get what the time differences were, you could look at running
ntpdate with the query switch
-q. Try it and see whether it is spitting out the right info for your purposes. From the manpage:
-q Query only - don't set the clock.
Or there is the debug switch
-d which may print out more info that you need.