I had to remove a few pacemaker batteries when I worked at a crematory. We threw them in bin in the processing room and until they ran out of juice, they'd beep like crazy periodically. They're made like that so that while the person is alive, they'll know when it's disconnected.
It was drilled into our heads that the crematory retort would be RUINED if one of those things went in with the person.
edit: typo and more context.
Whether or not they have a pacemaker is something we would usually find out by asking the family, nurses present at removal, etc. If none of those people knew, it's usually pretty obvious. The battery sits on the chest just under the skin and it's hard to miss. Just two or three incisions is all it takes to remove. It's easy to get shocked disconnecting the wires if you aren't careful though.
Cremation is quite permanent and mistakes can't be resolved easily, or cheaply, so cremation cases are inspected VERY thoroughly before going into the retort.