How can I determine if a typical low-end telescope is a refractor or reflector? Is there a way to distinguish (with no manual, product info etc)?
There is a wide variety of telescopes, so it's a bit difficult to give a general answer, but here are some pointers:

If there is an open-end, it likely is a reflector telescope (perhaps a Newton). In this case, you can see the secondary mirror attached inside.

When you look through the place where you set the eyepiece, are you facing the same direction as the telescope? If not, it is a reflector. 

Look into the part that you would normally point towards what you are trying to observe. If you can see a mirror at the opposite end, it is a reflector. 

On certain reflectors, there is a lens on the side that you point towards what you want to observe. In this case, there also is a smaller mirror attached to the inside of this lens. You can't see the reflective part of this smaller mirror, but there should be a couple of screws on the back, that can be used to align the mirror with the rest of the telescope. 

On a refractor, the part that you point towards what you want to observe is a lens that should have a rather round surface (as opposed to flat). This lens usually is not at the end of the tube, but rather there is a part to protect from stray light. 

It the tube tapers a bit, it most likely is a refractor.