Very broadly speaking, macro lenses have two things in common. The first is that their minimal focal distance, ie the distance from end of the lens to the subject, is usually fairly small so you can creep right up on your target. Secondly, the combination of the focal distance and the focal length on macro lenses means that you can get "life sized" images on the sensor/film. A good chunk of macro lenses are actually 1:2 though, meaning they can make the image on the sensor half the size of the true size of the subject.
In the description of the lens, the numbers you're looking for are where it says 1:1, 1:2, 1:3 and the word "Macro" to designate a macro lens.
I don't know if it's true on other bodies, but the majority of what's available as true macro lenses for Pentax are prime lenses. For strictly digital, the best option is:
SMC Pentax-DA 35mm F2.8 Limited Macro (For Pentax, the Limited means it's just a little bit awesome, metal bodies, best optics, etc)
There's also two full frame prime autofocus lenses that are macro:
SMC Pentax-D FA 100mm F2.8 Macro
SMC Pentax-D FA 50mm F2.8 Macro
Of course, if you aren't hung up on it also being an autofocus lens or a Pentax branded lens, there are a ton of options:
Macro Search at Pentax Forum's lens database