Well, are you looking online or at a store? Physically they look very different.
The easiest way to tell is size. The biggest tube sold in the US is Sony's 40". That TV and anything smaller is a direct view. Anything larger (43" and up) is an RPTV.
A rear-projection TV uses three separate, small (7"-9"), monochrome CRT's (red, green, and blue). Their light gets magnified by lenses and is then reflected off a large mirror, housed inside a cabinent. The light then strikes a set of thin screens where the image is focused. Rear-projection is used to achieve large screen sizes, typiclaly above 40"....
Direct-view TV's are just that - You look directly at a large, single CRT, which is coated with individual dots of red, green, and blue phosphors. Direct-view CRT's are limited in size (40" is the largest), due to the extreme weight, since nothing is magnified from smaller tubes, as is the case with rear-projection TV's. Direct-view TV's typically have thick glass in front of the CRT. Your computer monitor is an example of a direct-view CRT.