SMS Query for Service Pack

I need to run a query to find out what SP is installed on my fourhundred WINNT 4.0 PC’s. Some have SP4 some have 6a. I have identified files like WINVER.EXE which has a date of 10-13-1996 for SP4 and 11-18-1999 for SP6a, but when I query on that date all 400 PC’s come back with the earlier file(same)date. The same thing happens with any other file. Is there a better way? If so, what was your method?