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Contamination of flour is a serious problem in 'health-food' shops in th UK,
 and I would suspect elsewhere in the world.

As part of my research I visited a number of flour mills and contacted many
 more.
The conclusions are as follows:
Flour milled in third world countries is likely to be contaminated with wheat
 flour or worse.
Flour milled in traditional stone mills is likely to be heavily contaminated
 (unless only gluten-free flour is milled - but I have found no examples).
 All the mills I have contacted mill a range of grain in sequence.  The
 equipment is never cleaned between batches.

Even modern mills often run batches of gram or corn between batches of wheat.
 This produces several tons of contaminated flour each time the batch changes.

In the UK I have located only one mill that runs a dedicated gluten-free mill.
 That is Virani Foods at Wellingborough, Northants. They will not buy grain from
 third world countries because of the risk of contamination of the grain and
 high pesticide residues.
Most of their gluten-free grains are grown for them in Australia.I buy their
 flour by mail order and have had a very good service.

I would be interested to hear reports on mills in other parts of the world.
 You need to have a tour of the mill.
The mill should be dust free!
All hoppers should be enclosed.
Loading hoppers, augers, mills and bagging plant should all be separate and
 wheat free.

If in doubt, mill your own grain. I use a grain-mill attachment on a kenwood
 chef mixer. Freshly milled flour bakes better than stale flour.

Peter
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http://www.demon.co.uk/webguides/
Gluten-free Cookery The Complete Guide for Gluten-free and Wheat-free Diets.