I find this helpful knowledge to glass collecting . Most of your older glass is not marked  and is usually heavier than modern day glass. Some ealier though are blown and is thinner. Most mold marks are two anything made after 1880 when the Industrial age took hold. Older glass has wear to the bottom of the base which is not even. Those who have forged glassware to be old has taken a grinder to base which would be deep marks and are even in the scraped . Three and four mold marks are prior to 1880 . Your better glassware will have an oily feel to it and have ground bottoms .  This like Heisey, Fostoria, Cambridge, New Martinsville and Tiffin . The only pieced I have handled of Tifffany rang when it thunder out and loud noises this is true with lead crystal. If you buy a piece of pressed glass that is a light purple it has been exposed to the sun . Most companies like Fenton , Wesr moreland, Imperial did not mark there glass til 1960'-1970's  I hope you find this helpful a older antique dealer of glass and reading of books have taught me this basic knowledge .Good books I have found on glassware Lee Weathermans , Fenton the First Seventy Five Years, Mauzey's Depression Glass this tells you of remakes ,Warmans pressed glass ware book Anything by Florence I have found very helpful as well of the different companies books or advertising .