Pneumatic Vs Mechanical Rotary Hammer
I think the first step is to clear up the terms you are using.  There are "rotary hammers" and there are "hammer drills".  You are combining the two, causing some confusion.

In general, if a product is called a hammer drill, it will have (to simplify) a pair of ridged disks inside it that rub together and cause the bit to move in and out a little when the drill rotates.   This hammer drill will have a high "beats per minute" and has a characteristic sound almost like slowly rubbing a coarse file with a nail.

A rotary hammer is normally larger and the in and out motion of the bit is performed by a pneumatic piston.  They are more powerful and also absorb shock more for the user so can be used for longer periods.  They also will typically have the ability to use the hammer without the bit rotating (which is not possible on a hammer drill).

So to answer your last question, if it is called a hammer drill, it uses mechanical percussion, and if it's called a rotary hammer it uses a piston.  I guess a manufacturer could use the wrong term and at that point you would need to look at the manual or a parts diagram. This article from Popular Mechanics actually mentions a drill that is marketed as a hammer drill but is designed like a rotary hammer (the Rockwell).