How to know if DMA on STM32 is being used or not
This is a rather silly question - you wrote a program designed to use DMA, and the fact that you don't have any other code to perform this should be evidence enough that the DMA engine is doing it... if it is actually happening.
Formally, if you really want to know, you should study the source code of the vendor HAL routines you are calling to configure and start the DMA.  The HAL routines are typically compiled from source alongside your custom source code, and they have halfway decent comments, so no great mystery.
I suppose if you really wanted to &quot;prove&quot; it was the DMA engine responsible, you could go and make one tiny edit to the HAL DMA function so that it didn't actually activate the DMA peripheral, but only went through 99% of the motions of preparing it.  Do a full clean and rebuild and reflash, and you won't see a progression of data coming out as varied pulse widths.  Or you could do something like change the source increment set into the DMA hardware alone, and so cause it to skip every other value.
Or yes, if you want, you could use the debugger and a study of the programmer's manual to go and do a special function register write which would disable the DMA, or kill the clock to it or whatever, after the program had enabled it.  Just make sure the program isn't doing operations in a loop which might cause the DMA to be quickly re-enabled again.