A simple and easy way for anybody to quickly assess whether or not a piece of glass is handmade is to look for the tell tale signs in the appearance.
The first thing to look for are cutting marks or rod marks that signify the place where the glass was attached to the blowpipe and cut off. Items like wine glasses or vases would have such marks visible underneath the base. Depending on the shape of the item there may be pinch marks, for example on handles or lips of jugs.
Next raise the glass up towards the light and check for imperfections such as small bubbles, uneven surface or slight irregularity in shape. Bubbles occur naturally during the blowing process when air gets trapped in the molten glass but sometimes the artist purposefully adds bubbles to the glass. Since machine-made glass can also have bubbles, bubbles by themselves are not strong enough indicator that glass is handmade. If possible, compare two items of the same kind - even the most talented artists who achieve near symmetry never produce pieces that look 100% identical.
Handmade glass often has a discreetly located makers mark. Do not confuse a signature with a mark pressed onto the glass, the latter usually indicating that the glass was machine-made. Always check for other characteristics of machine-made glass such as seams along the sides marking the place where the two sides of the mold were assembled.
A significant amount of skill, craftsmanship and experience is required to produce handmade glass; the imperfections in glass make it interesting and unique, perhaps explaining why the interest in handmade glass hasn't diminished.