If the name of a plant includes the full species name (like Asclepias tuberosa) then it is the real deal. A plant that contains the species name plus a name in quotes (Asclepias tuberosa 'Hello Yellow' or Lobelia cardinalis 'Shrimp Salad') means that it is a cultivated variation of the species plant. This plant will be vegetatively propagated to continue the trait for which it was selected (in these two cases, color).

If however you find a name with an "X" in it then it is a hybrid and is the result of two different plants. For example, Lobelia x 'Ruby Slippers' is a hybrid (note the "x" after the genus name). It is a hybrid between two native plants, L. cardinalis and L. siphilitica. It is sometimes shown as Lobelia 'Ruby Slippers'. Note the lack of species name.

Sometime hybrids are with non-native plants: The Cornus genus has been hybridized in several ways with the native flowering dogwood, the Asian dogwood and with the native Pacific dogwood:
- Hybrids with Cornus florida (eastern flowering) and Cornus kousa (asian dogwood): Cornus x ÂConstellation sometimes noted Cornus x rutgersensis 'Constellation'
- Hybrids with Cornus florida and Cornus nuttallii (Pacific dogwood): Cornus x 'Eddie's White Wonder'
- Hybrids with Cornus nuttallii and Cornus kousa: Cornus x ÂVenusÂ.

YouÂll note that when it comes to hybrids, the nursery tag does not always present enough information to know if the hybrid has native parents or includes exotic parentage.

And just to make it a bit more interesting, occasionally a hybrid can be made between plants that are not in the same genus. The plant "x Chitalpa tashkentensis" is a cross between Catalpa bignonioides and Chilopsis linearis, both in the Bignoniaceae family. DonÂt let the exotic name fool you - Chitalpa is a combination of the scientific name of the two parents (both natives to the US), while the specific name identifies the city in Uzbekistan where the hybrid was created. This hybrid now has two cultivars, 'Pink Dawn' and 'Morning Cloud.'

The best and easiest way to tell if a plant is a native US plant is to use the search (on left side) at the USDA site linked below. Just use the Genus or the genus and the species name. Generally additional names unless they are identified as variety are not considered anything other than an horticultural variant of the species.
posting in Asclepias tuberosa into the search opened up a page with Asclepias tuberosa listed along with several subspecies names. Just click on those names that are links to see images, a map of distrubituion and other information and relevant links.

Here is a link that might be useful: USDA DATABASE

I think if you want the authentic wild strain of a native plant you need to be very careful. As the others have said, a lot of what you buy in nurseries includes selected cultivars of natives, hybrids, etc. Another problem is that even a wild strain of a native plant might not be the the same variety that grows in your locality naturally. Many native plants occur over a really wide geographic range and the genetic makeup of the plant might differ greatly. I like to try to grow local strains of native plants, which generally means collecting seed or starts from wild population and propagating them myself. It is more work to do it this way, but I think its worth it.