I think it's hard to be sure, unless you provide the oil and then watch the tech stick it in the crankcase.
You can't even be sure you're getting the right "dead dino" oil when you go to the dealer. Your car manufacturer may require 5W40 but the dealer may buy 10W30 in bulk and pump it into every car that comes in the shop.
What you could do is have the oil changed and then immediately have a sample sent off for analysis. Get a witness or two, put it on video or do whatever it takes to prove that you're not just setting the dealer up.
If the oil analysis shows that the oil is made up of a base stock other than a synthetic one, you could then take all your info to your state attorney general's office and get the dealer in a nice batch of hot water. Otherwise, you'll get peace of mind. Trust, but verify. :-)
An oil analysis kit should cost around $20.