William Weber states that there are two keys to distinguishing between the two species: 

1) M. bigelovii's peduncles (the common stalk of the flower cluster) are very sticky; M. canescens' are not.  In the photograph below of M. bigelovii, you can see the gleaming dots of the sticky, glandular hairs covering the phyllaries, flower stem, and small leaf-like structures.  You can also see small buff-colored sand particles stuck to these hairs.  About half-way up the stem on the right side, a black ant is feeding on the sticky sweetness. 

2) Both M. bigelovii and M. canescens have phyllaries in over-lapping rows with curved tips, but M. bigelovii's phyllaries (left photograph below) are green at the tip and for at least half the length of the phyllary.  M. canescens' phyllaries are green only at the tip and the lower three-fourths of the phyllary is tan/white (often with a very narrow green stripe in the center).  A hand lens will help you observe the phyllary characteristics in the field.

I find that many plants mix the above characteristics: some have sticky peduncles (M. bigelovii) but phyllaries green only at the tip (M. canescens).

Phyllaries of both pants are sticky and you will often find various wind-blown particles stuck to the phyllaries.  See especially the left photograph for the sand particles stuck to the flower stem and phyllaries.  The slightly toothed leaves of both plants are narrow, but those of M. bigelovii are usually narrower.

M. bigelovii is usually a bit taller (but both plants can exceed three feet), less branched, and has fewer flowers.  M. bigelovii flowers tend to be deep violet to blue-white; M. canescens are lavender-pink.  M. canescens can be a crumply, raggedy plant with flower petals curling like wood shavings from a carpenter’s plane.  M. canescens stems are rough, not sticky, and sometimes brown-red and stiff.

In most years both Machaeranthera species put on magnificent flower displays in foothill fields and along roadsides.  They are also very long lasting, often into late October.

"Machaer" is Greek for "sword" and "anthera" is Greek for "anthers" from "anthos", "flower".  "Machaeranthera" thus refers to the sword shape of the anther tips.

Foothills, montane, subalpine.  Disturbed areas, openings.  Summer, fall.
Bisti/De-Na-Zin Wilderness, October 8, 2007.

M. bigelovii is common in fields and along trails and roadsides in late summer and fall, sometimes occurring by the thousands in very attractive displays.  The plant begins growing in early summer and often grows to over two feet tall (as in the photograph at left) with a strong, almost woody, central stem.  Leaves are few, short, and slender and often only on the upper part of the stem by flowering time.  It is usually the magenta flowers, seemingly floating, that attract attention.  This species occurs almost exclusively in Colorado, New Mexico, and Arizona.  M. canescens (below) is widespread through all western states.

Dr. John Bigelow was a Professor of Botany who collected in the Pacific Railroad Survey of 1853-1854 in the West.  (More biographical information.)

Foothills, montane, subalpine.  Disturbed areas, openings.  Summer, fall.
Hunter Canyon Trail, near Moab, Utah, September 26, 2005.

Foothills, montane, subalpine.  Disturbed areas, openings.  Summer, fall.
Big Spring Trail, Canyonlands National Park, Utah, September 10, 2005.

Foothills, montane, subalpine.  Disturbed areas, openings.  Summer, fall.
Near Canyons of the Ancients National Monument, August 28, 2005.

Machaeranthera canescens is a complex species with many variations.  This photo shows a typical plant at full height with numerous flowers.

"Canescens" is Latin for "becoming gray".

Thomas Nuttall collected the first specimen of this plant for science on the banks of the Missouri in 1811 and it was first named Aster canescens by Frederick Pursh in 1814.  Asa Gray named it Machaeranthera canescens in 1852 and the plant has endured several dozen other name changes since then.

Foothills, montane, subalpine.  Disturbed areas, openings.  Summer, fall.
Near Canyons of the Ancients National Monument, August 28, 2005.