The easiest way that I know to tell is the shape of the dash panel. The 155 has a platform that juts out of the dash, steering column and the travel control lever come up through it, the detent for the travel control is located under this platform. The 150 doesn’t have this platform and the detent for the travel control is a metal bracket kind of thing that's out in the open.
If you go to the history tab on the front page then the second set down where is says, "Case 100 Series Garden Tractors - The Desert Sunset and Flambeau Red Years."
Photo number 24, there is a 195 in the upper left corner and you can see the platform thingy I'm talking about.
ByCo