Hello dxphoto,
I would tend to agree with the underexposed transparency film, though it is tough to tell when viewing JPEGs on the internet. Since I have seen some published images from Alex Webb, I do think Magnum have done a good job represecting his images on their website, though you should click on the higer resolution link below each image to get a better view.
One thing that can help is figuring out the time of day. If you see long shadows in the images, then it would need to be early or late in the day. A lack of shadows can mean closer to noon or midday. Shooting at midday can also create some higher contrast results with some transparency films. The comments other made about lights give other clues, since street lights can alter the results on film, even though you would not normally notice those with your eyes.
Another clue is that the images are not super saturated, so it would be less likely he used something like Fuji Velvia or Kodak E100VS. While Kodachrome is one option, if you don't like the expense nor slower turnaround time (in some parts of the world), then try something like Fuji Astia 100F or Kodak E100G to start.
Ciao!
Gordon