There's a bunch of things...the Hans G. Lehman photographs that kick off this thread are id-ed as BMW prototypes. If you quote the first post you'll see the provenance of the photos. Some of the images have popped up on other dedicated websites....World Car Fans, for example, that are very reliable identifying so called "spy photos".
The "M" designation on some of the license plates stands for "Munich"...home of BMW.
All the prototypes wear a "fat-suit"- an intricate 2nd exterior skin of camouflage that covers the entire body. I've seen other manufacturers cover parts of the body but no one except BMW goes to such trouble as to cover the entire exterior with the "real" skin underneath.
Take a look at the passenger side rear shots where you can see the gas tank cover on the real car underneath.
Some tricky stuff.
On one of the rear shots you can clearly see that this is running on a 1 series platform...the beefed-up double wishbones are the most obvious clue.
As Harold mentioned you can see the BMW proportion...the extremely short front overhang, the comparatively long wheelbase for a car this size...
all of it points to it being a BMW.