Luck?
I usually start off on the weed galleries but sometimes I never find what I'm looking for. A lot of times I get lucky with a descriptive word on google.
There is a method that we learned in one of our master gardener classes that I've never tried and have to look for. It follows a series of questions and answers which progressively narrows down your choices. I don't know how all knowing it is and to what degree it works. I'll try to look it up.

Loretta:

Are the leaves opposite or alternate, entire or toothed - that kind of thing? Allegedly it works - fortunately for me I already knew a great deal about plants when I took botany so when it came time to ID a plant for a test, I normally had a pretty good idea of what it was 'cause anytime I tried the system above I was not successful.

Horsetails aren't in the wildflower books because they aren't flowering plants - they reproduce with spores, not seeds. And they aren't ferns, either, they've got their own family. So they won't be in any book that only does ferns. So, in answer to your question:

1) Luck
2) Get a copy of Gleason and Cronquist's 'Manual of Vascular Plants', THE technical reference work for this area. It's expensive - and if you want pictures you have to shell out for the even more expensive 'Illustrated Companion to Gleason and Cronquist'.
3) Get a fern book that identifies other non-flowering plants - 'Ferns and Fern Allies' or the like. That should include horsetails and clubmosses, at least. Truthfully, when I'm really stumped I still pull out my ancient (1967) little Golden Nature Guide 'Non Flowering Plants: Ferns, Mosses, Lichens, Mushrooms, and other fungi'. I think it's still in print. Excellent illustrations.
4) Look in really local plant guides: if your local state park or nature center puts out a 'guide to plants in our park' they will often include horsetails, because they look so striking that people notice them (and they're interesting - very ancient, different way of reproducing, silica in the stems, etc.)
5) Ask somebody who knows more than you do (hey, it works for computers!)

See, more than you wanted to know.

WW

Take a photo of it and post it on the Name That Plant forum on the Gardenweb. They have very knowledgable folks who can identify almost any plant.

It's funny that you came across some along the roadside. I had to buy some from Tripple Brook Farm! http://www.tripplebrookfarm.com/iplants/index.html
Equisetum hyemale robustum: the giant scouring rush

Mucknmire, they are all along about a 1/4 mile length of the road - however, like lots of the wildflowers, specifically colts foot and spring cress, there seem to be fewer this year than in the past.

Yes that is the system. Thought it sounded too good to be true. I wanted to try it against that plant contest they use to hold here but I don't see any old reference to it. I'll have to find one of those back in time links I saw posted here somewhere.

Weedwomen, those sound like some interesting look-ups.
I was hoping to go through the references they have at hand at the extension center but it is not easy to get a spot covering the helpline (yet it is a requirement - go figure).

Geez, maybe I should cancel my order and look around in ditches! I was looking for the large variety though to use as an architectural planting. You're lucky to live where there are lots of wildflowers. I don't see very many in Spring where I live or maybe I'm not as observant as you are. Probably both.

don't be in a rush to put this in your yard or pond. It spreads everywhere and pops up just where you don't want it. Grow it in a container if you must have it. I have spent years pulling it out.

I just saw it for sale today for around $50 for a large container. That was pretty nervy I thought.

tiarella, thanks for the warning. I intend to keep it contained. I saw some planted in a trough about 8" by maybe 5 feet wide for an entry. It was quite a striking display so I want to try to recreate it.

$50 loretta? Pretty nervy indeed but some plant novice will be suckered by its unusual look, sort of like bamboo without leaves in some stages of growth. Besides, I don't consider myself to be a novice but I still bought some but not for $50! And it was for the giant variety.