It is a little hard to say. You can send a sample to Blackstone but that costs a little bit. If you were suspicious BEFORE they did the work then there might be some things you could do. I had a friend with the same suspicion. I looked at her car and the oil filter cap had undisturbed dirt film on it. And the oil looked pretty dark for new oil, but that is not a scientific test. This is why I try to do all my own work. I figure I will screw it up some times, but still less than a "pro" will screw it up, and then at least I know who to blame, and I know exactly what was done and what wasn't done. That is the value of DIY.

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra