If someone were to have had, for example, 338 btc at MtGox and lost it, how will they be able to file that on taxes now? How can we prove that we had it to begin with if it's gone now? According to what I have read, the IRS is treating btc as property vs currency which would put it into capital gains. Would someone just file it as 338*600 capital gain loss for 2014? What type of documentation can/would we/they need to provide to do this?

Thanks