I’m going to attempt to answer your question for real. I have never had to cremate a person, but I have cremated pets before. One time I think I may have gotten a symbolic collection of ashes, and the other time I was pretty sure I got my exact cat. The difference was visible in their system.
The first time I did not ask many questions, and I ended up with a bag of ashes that had a sticky note with my name on it. It would have been very easy for the wrong bag to be placed with my name, or for the notes to fall off and get mixed up. It was clear that accuracy was secondary to creating a chance for closure. They were very nice and professional, it was just clear that they had not felt it necessary to have an iron clad ash delivery system.
The second time I needed to cremate a pet, I asked a lot more questions, and all were answered without any surprise. Considering the type of business it is, it is always ok to ask a lot of questions about the process.
They put a tracking barcode on my deceased kitty as well as on the body bag he was placed in. When I picked up the ashes days later, the same barcodes were on the tightly sealed bag as well as on the carrying bag, plus they had his collar and a pawprint memorial in the bag too. They could have given me random ashes still, but the care that they clearly put into their system gave me a strong feeling that they had held up their end of the bargain.
The sad truth is that there is probably no way to be 100% sure, and it is likely normal for some ashes to get left behind while others may be unintentionally scooped in. The best you can do is make sure that you ask all the questions you need to (don’t let anxiety shut you up), and try to pick a place that will treat your loved one with dignity.
The place I sent my parrot to when he passed puts a numbered metal tag on each pet before they go in and the tag stays with the ashes. They also did a foot impression and sent the ashes back in a simple but lovely clay urn. It was a little weird seeing the ashes in a ziplock inside the urn but I totally get it since they were a small family business.