For the GLD, SLV and other precious metal bullion ETF's, it is kind of easy, with the Gold and Silver prices readily available and unambiguous.
For Miner ETF's such as GDX and the like, it is less evident. Some ETFs (such as GDXJ, GLDX) have created a benchmark of their own. The GDX can be benchmarked against the large cap precious metals miners indices available: The Philadelphia Gold & Silver miners index (XAU) or the AMEX Unhedged Gold Bugs Index (HUI).
Gold Mining investors have been badly beaten up lately. The XAU is considerably lower now than what it was like about a year ago (220-230), with gold ranging around $1460 around this date of last year. But how well is the XAU mining index? I've tried to answer that question by making a benchmark of the XAU against the HUI index.
Read the conclusions in the posting below: http://gwyde.blogspot.com/2012/04/benchmarking-hui-versus-xau.html