If they are 'Fair Trade' coffee then this means they have a Fair Trade certification and will say on the coffee bag, or at least be advertised by the seller as so. All you need to do is ask. However, to know if something is actually traded fairly or not is a different situation. 'Fair Trade' as you know it is still a for-profit company which, while they rise the min payment to the farmers above what coffee is typically bought for without this minimum in place, it still is a very small number (as of April, 2011 the FT price set is: 1.35$ for conventional natural, 1.40$ conventional washed, 1.65$ organic natural, and 1.70$ organic washed) compared to the actual retail value of coffee (around 10-14$/lb in the US). And Fair Trade still works for profit so they still take part of this value for themselves in order to try and give more to others. i.e. it costs about 3,500USD a year for one co-op in Laos to keep their certification and this money of course goes to the organization 'Fair Trade'.
In a situation where farmers of coffee make pennies and the 'middle men', buyers, auctions, certifications, and other organizations like Fair trade take money and value away in order to make more money for these 'middle men'.
So, it dosen't seem you will be able to know for sure. Hopefully this can change and we as consumers, can start to communicate and work with the farmers and communities themselves...