I bought a tablet computer with the intent of using it only for work purposes. What's the best practice for proving the IRS that I only use it for work?

If you are self employed and the computer is generally accepted as something used in your business you don’t have to prove anything. 

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@Bsch4477 wrote:

If you are self employed and the computer is generally accepted as something used in your business you don’t have to prove anything. 

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I think it would be very rare for the IRS--even if they audit you--to ask to see the tablet and look for apps like Netflix or Hulu or mobile games.   (The IRS does sometimes do full compliance audits where they ask to see every piece of paper and account for every dollar in your bank account, but these are extremely uncommon, and they are used to build up a statistical picture of how compliant the US is as a whole, rather than an enforcement action on an individual.)