If a dryer circuit was installed before 1996 there is a good chance it won't be grounded. Before 1996 the US national electrical code allowed 3 wire dryer circuits. Two hot wires and one neutral.
In 1996 that was changed to require new dryer circuits to be 4 wire. Two hots wires, one neutral, and one ground. Existing 3 wire dryer circuits were allowed to remain in use.
Regardless of being grandfathered in (allowed), you should update your drier to a 4-prong cord/outlet (which is ACTUALLY grounded). This does not always require stringing an entire new circuit — NEC (US) allows driers to be grounded with just a single 10awg wire.
Particularly if you operate rentals, the legal liability is perhaps almost as much as NOT retrofitting GFCIs into baths/kitchens.
In 1996 that was changed to require new dryer circuits to be 4 wire. Two hots wires, one neutral, and one ground. Existing 3 wire dryer circuits were allowed to remain in use.