Hopefully they don't send you an inactive enantiomer!

Or an insoluble polymorph.

Realistically, a lot of cancer drugs are injectables, ramping up the risk of something going awry when you inject the powder you will have to handle.

I'm guessing the people who can afford a mass spectrometer can already afford the drug (or at least better insurance)

Heh, well if the drug is $4k a month and you can get the equivalent 5 year supply for $100 that leaves quite a bit of money left over for testing.

There are lab services that will run samples through a mass spec and report the results.