This site may be of some use: http://www.audiotubes.com/mullcode.htm
I can only guess you're talking about real Mullards, not the recent New Sensor production with "Mullard" labels. The good old stuff can be confusing with all of the different labels (and re-labels, and sometimes re-re-labels!!) they put on them. But there should be a date code, and the site above should be able to help you with that.
Another little chart that might help: http://home.carolina.rr.com/billdougan/Bias.htm
If you have the Mullards in hand, you can always put them in an amp and fire them up to see if they are Mullards or not. A real Mullard preamp tube will give a bright flash when the amp is first turned on, then die down quickly to a normal glow. That doesn't narrow it down completely because some of the other Phillips tubes do that as well. But you should be able to see an "I61" or "I63" or something depressed into the glass.
There are lots of pictures and information available on the internet so if you're not sure yet, it's time to do some research before spending anything on these valuable tubes.