I visited Marfa a few weeks ago and other than the remnants of the main gate, there's little if anything remaining to view from the road. The site is in fact on fenced private property, but conveniently a very nice viewing platform with telescopes has been erected due north of the site along Highway 90 to provide viewing of the "Marfa Lights" (a phenomenon along the distant mountain range that been documented as far back as the mid-1880's). Using those binocs you can see two distinct concrete foundations that still remain, the basement under one of which reportedly still has artwork applied by German POW's while they were interned there during the war.
I was actually kind of disappointed since the Google Earth image shows what would appear to be a fairly intact base with roads, structures, and runways still clearly outlined. Almost none of this is visible from the road, even from a slightly elevated position with binocs looking over essentially flat terrain.
As a side note, if you look roughly 45 degrees to the right of the base from that position, you'll be looking at the movie set where the outdoor portions of "There Will Be Blood" was filmed. It's about 7 miles away, deep in the desert and well away from the road, so there's nothing really to see, but its there FWIW.

BEAUTIFUL cointryside, especially once you being traveling south toward the Presidio border region.
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Rob Mears
'Surviving Corsairs' Historian
robcmears@yahoo.comhttp://www.robmears.com