There are obvious tells in almost all of them -- mostly lowercase "s", "c", etc, in which Helvetica is has perfectly level edges and arial is angled.
The only harder ones are some of the all caps examples like TOYOTA.
For uppercase, the tip offs are the following:
* The capital A in Helvetica is narrower (more isoceles and less equilateral)
* The capital G has an extra hatch on the right side (looks like an arrow and not an L)
* The capital R does not have a straight leg in Helvetica
* Conversely, arial chooses a non-straight hatch mark for the Q whereas Helvetica's Q hatch is straight.
This image provides a good overview on the capital (and numeric) differences: http://cdn.ilovetypography.com/img/gqr.gif
In the case of TOYOTA where it seems that kerning might different in two images, the heavier strokes in Helvetica should tip that off.
That said, I did get a degree in graphic design, so I may not be typical when it comes to this.
I notice the differences (slanted terminals, narrower "A"s, etc.), but had no clue which was which. Weirdly, I'm super-picky about typefaces, but mostly in a "I like what I like" sense rather than a "I know the precise details about why" sense. I dislike both Helvetica and Arial intensely, and so never bothered to learn much about what made them different. They're both just fonts I don't choose for anything.
This quizz is an excellent training though. I didn't know anything at all about typography prior to taking the test (never bothered to learn the names of the fonts or their shapes), ended up scoring 17/20.
http://snth.net/~rlk/image/helvetica-vs-arial-2.png
In this comparison, the capital E's are identical, so perhaps MATTEL used a different weight Helvetica?
I couldn't see any difference between Mattel and Toyota.
Mattel was also the one that tripped me up.
As the GP said, the caps on the S, C, T, etc were usually the giveaways.
The flourish at the bottom right (not sure the correct term) of the R in TARGET gave me at moment's pause. But each of the logos was customized to an extent.
It's funny, because I universally prefer Helvetica over Arial otherwise.
Lowercase Helvetica characters never end on an angle. They are always flat and either parallel on the bottom or the edge of the screen.
Toyota was a pure guess for me.
I guess that anyone who knows anything about typography would answer correctly to all questions.
Clue: I went for bolder font. Also, Helvetica is a bit wider in these logos. After that, I also noticed the clue in C.
All the rest I had immediately, on the very first image there was 50% chance I had it right and I did a lucky guess, from then on I knew Helvitica was horizontal, Arial slanted.