Many Glass items are listed as "Mouth Blown", or simply "Blown".

Suffice to say that the vast majority of these are neither. We are fully aware of the common usage aspect of calling any fine example of Art Glass "Blown", but one shouldn't go too far by calling it "Mouth Blown". It refers to the method of how the item was made, and when one sells it, one should be reasonably accurate in one's description.

So how do we tell what is what? Here is a quick thumbnail sketch of the most common practices of creating a piece of shaped glass:
1. there is molding, whereby molten glass is poured into a cavity of the shape desired. Once solidified, the mold is opened and out comes the molded part.
2. "Mouth Blown" or in short "Blown" employs a hollow tube of heated, pliable glass, which is closed at one end. Upon blowing into the tube, one can make it expand, somewhat akin to blowing up a balloon.
3. Flame worked glass starts with heated, pliable piece of solid glass, which is transformed into shape with tools of some kind, with the continous aid of a torch flame to keep the piece pliable.

As for item 1., most paperweights, nuggets, coasters and trinkets are done this way. Items tend to be heavy, solid pieces of glass.

As for 3., most all shaped glass figurines and fanciful designs are done this way. It is by far the most common practice of creating glass items.

To spend a little more time on our topic # 2., the "Blown" glass mystery, let's revert back to our analogy of blowing up a balloon. The process presumes a closed hollow shape, the open end being the part where one has "blown" into. One can then cut off the expanded roundish shape, looking something like a pumpkin, and by reheating proceed to form and shape this hollow shape.

It is normal to cut it in half or portions thereof to make vases or bowls. These are simpler shapes and tend to be of heavier thickness, with minimal blowing at the outset. Finer hollow spheres will usually come in much thinner wall thickness, and allow intricate shaping due to the relatively easy heating of thin glass walls. Most of such designs will end up with a fully closed sphere, where at the end of the shaping process, the artist seals off the open part.
A quick note on the typical glass "Tree Ornament", these are now automated processes, with machine precision,  which  generate incredibly thin walls, albeit  by machine blowing in conjunction with molds.  Technically a blown item, but not accepted as such by the glass afacionado.

So when you look at an item, and it looks as if one might have pounded it to shape as from a piece of bread dough, or if it has fanciful contours and appendages all in solid glass..... it is not "blown".

If the item is an open shape, hollow sphere, bowl or ball shape, it was likely "blown". If the wall thickness is heavy, it probably had minimal blowing; if the wall thickness is quite thin, it would have started with thinner walled tube stock, but chances are it also had more intricate work done to it.

Claims of "blown" glass are supposed to lend an aura of respectability to a simple piece. That is probably not illegal, but perhaps now you can tell the difference.