How do you inspect a bitcoin transaction to avoid getting scammed by a double spender? I'm buying bitcoins in cash and the seller said I could inspect a transaction instead of waiting 6 blocks. Is this safe?
There are no means of inspection that could lead to a safe assumption that a transaction will not get invalidated by a double-spent input.
Giving up your money and leaving the trade with zero-confirmed coins is asking to get scammed.
Waiting for one confirmation is only safe if you are certain that the seller exchanged a total of less than 25 coins over the past few minutes (both in-person and online) and does not control 50% or more of the network's total hashing power (so it wouldn't cost him a penny to perform this kind of attack). Luckily for you the most a mining pool currently holds is 25% of the network's total hashing power (attacks are still possible at that rate but less likely to succeed and cost more for the attacker) but unfortunately you can never be sure that the seller or a partner-in-crime didn't spend more than 25 coins online so one confirmation won't do the trick either.
Waiting for more confirmations will always increase your security to a great extent. 6-blocks confirmations are not 100% safe either but they are way much more secure than single-block confirmations in all aspects.
On a side note, a reliable escrow service could take the hassle away and make your life easier when trading with strangers on the street.