This is one of the oldest tricks in the bar business. One way to tell if the bar is refilling premium bottles with the cheap stuff (or worse) is to take a look at the bottles behind tha bar: If you see torn, faded, or stained labels on popular premium brands, then there is a good chance that the bottles are being refilled with the generic stuff.
Another way to tell is to taste it. If a person can’t tell the difference between Jameson’s Irish Whiskey and Powers, then perhaps the person shouldn’t order the good stuff in the first place.
The second oldest trick inthe book is for the bartender to pour a “premium” drink below the bar out of the customer’s sight using the cheap stuff. This is a common practice in tourist type bars with a high turnover of non-repeat customers. If the drink doesn’t taste quite right, then ask for another in a clean glass and keep your eye on the bartender.
“...then perhaps the person shouldn’t order the good stuff in the first place.”
Jameson’s is good stuff? Bushmill’s, and especially Black Bush is far better than Jameson’s.