The easiest way to tell if you have a high- or low-profile blade holder is simply to measure its blade groove. High-profile blades are about 0.5 inches tall, while low-profile blades are 0.25 inches tall. The depth of the groove should tell you what type of holder you have.
Alternatively, you may also have an adjustable blade holder. These holders allow you to add the ledge to accommodate low-profile blades or remove it to use a high-profile blade. You can identify an adjustable blade holder by the two screws holding the ledge on; to remove the ledge, simply take the two screws out and slide the ledge off.