For example, these aircraft (737-700 and 737-800) have the vortex generators:
Photo © Mark McEwan
Photo © Ruud Brinks
And these (both 737-700) do not appear to have them.
Photo © Juan Carlos Guerra - FlyAPM
Photo © Andrés Contador - AirTeamImages
Re the Aeromexico 737-700 above, there are photos of other AM 737-700s that do have those vortex generators. One below. Why the difference?
Photo © Matthew C. Lyons
Another general question: Why are these types of vortex generators much more common on Boeing than on Airbus aircraft? It seems that almost all Boeings (except possibly the 747; I'm not sure about the 787) have many of these protruberances.
I was on a KLM 737-400 a couple of weeks ago, seated just in front of the wing, and counted 26 vortex generators on each wing and another 4 or 5 on the inner side of the engine cowl. There are also 4 on each side of the rear fuselage between the vertical and horizontal stabilizers, for a total of about 70.