the bottom line here is if you want high quality silk then places like Jim Thomson's is the place to buy, but not at retail prices.
Unbeknownst to most visitors there are 2 factory outlets selling home furnishing fabrics, out-of-collection silks, cotton and linen; fabric by the metre and remnants. Prices are significantly reduced when compared to their retail outlets in the glitzy malls.
The 2 outlets are located at 9 Surawong Rd, Silom and at 153 Sukhumvit Rd Soi 93. I've virtually purchased most of my stuff from either of these 2 places.
Or if you have time the Jim Thomson Silk Farm is located about 2 hrs NE of Bangkok near Korat.
There is also a once a year clearance sale held in December, usually at BITEC, even cheaper than the outlets.
2 things i need to point out
1. the so called light test and other tests for silk that SnowPirate mentioned does not cover all types of silk
2. China town predominantly has lowewr quality silks mainly brought in from China. Most of the tailor shops geared towards tourists buy their stuff from here and then claim it as good/high quality.
bkk3