1,4 is easy to tell. It solidifies at less than 60 degrees. Actually its a solid at higher temps than that, but I always test it at 60. At room temp is has a syrupy consistancy.
another easy way to test would be to mix a teaspoon of NaOH, or KOH, 1 tblspoon of the GBL, and 1 tbsp of water. Look for exothermic activity. If there is none, you got bunk stuff.
GBL is so cheap in most countries (even in the US....but now you need a permit to buy it) that there's no reason to fake it.