All the above advice is great! I love the inventiveness and resourcefulness of Pattern People!
In desperation once, I looked throughout the pattern itself, even the INSIDE of the envelope! But, I finally did find the date! It was printed in the margin of one of the instruction sheets! I can't remember what pattern company, unfortunately. Maybe Butterick. But try that as well.
Also, along the same lines as Melanie mentioned, check out the postures of the models. If the hips are thrown forward, perhaps it's from the 50's. And in some of McCall's illustrations from the 70's the women have proportionately larger feet, possibly influenced by the "keep on truckin' "
posters.
And take advantage of the "circa" notation. The standard convention for its use is that it is used as an indicator of a 10 year range. So if I note "ca. 1964", it implies a date range of 1959 -1969, if my math skills are correct!
Good luck! And keep on truckin'! :O) This can be a frustrating task.