In two words, you can't. Pictures lie and, unfortunately so do people selling bikes. I spent three years looking on ebay, craigslist, Walnects and cycle trader looking for a classic bike. I was tempted a few times when the deal seemed good to buy sight unseen. I actually took several long trips to look at bikes that were "perfect" only to find otherwise. All but one (the one I eventually bought locally) were misrepresented. Some unbelievably so. I won't take up everyone's time with the gory details.
After that experience, I'd never buy a bike or give up any money without a very close look first. If it's a newer bike, you might have the owner take it to a reputable dealer and get documentation it's what the owner says it is. Have it mailed to you by the dealer. This, of course would be an additional expense. Let the buyer beware.
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