Yep, definitely no way to know for the average driver. Even if
they show you a set of spark plugs and oil filter that was changed,
there's no way to know whether those were yours.
As for engine oil, most of the common engine oils are golden in
colour when new, but for all you know, they could have used a cheap
brand of oil when they charged you for OWS engine oil.
As mentioned, try to hang around your car so that the mechanics
cannot try anything different, but as for AD, too bad, your car is
in their hands for the whole day.. 
Bottom line is, try to find a workshop that you can trust..
