This is a very difficult question. Different botanists have described different numbers of varieties within this species, using different characteristics. No varieties at all are described in one of the most recent references, the Flora of North America, although this work does describe the problem. Quoting from the Flora of North America page linked below:

Achillea millefolium is morphologically variable and has been treated as either a single species with varieties or as multiple distinct species. At least 58 names have been used for North American specimens. Some early workers (e.g., J. Clausen et al. 1948) thought the native North American plants were taxonomically distinguishable from introduced, Old World plants. Other workers (e.g., R. J. Tyrl 1975) have treated A. millefolium as a cosmopolitan, Northern Hemisphere polyploid complex of native and introduced plants that have hybridized, forming diploid, tetraploid, pentaploid, hexaploid, septaploid, and octoploid plants and/or populations constituting a single, variable species.

An older work, Gleason and Cronquist's "Manual of Vascular Plants" makes a relatively simple distinction between native and introduced Achillea: "Typical European A. millefolium, a hexaploid with flat leaves, the relatively broad ultimate segments all in nearly the same plane, is rarely introduced in our range. Our native forms are mostly tetraploid, with narrower leaf-segments disposed in various planes so that the leaf is airily 3-dimensional. These represent the transcontinental ssp. lanulosa (Nutt.) Piper, consisting of several varieties as yet not well understood."

You could try contacting the company that prepares the wildflower mix and asking about the origin of their seed.

Here is a link that might be useful: Achillea millefolium in Flora of North America

Thanks for the help!
I would contact the company, but somebody threw away the seed packet, so I don't have the information anymore.
My yarrow seems to grow shorter than the wild type, so I'm going to (attempt to) eradicate it. Too bad the wild type is not reliably native.