Where the register is

Every regulator keeps a public register of the operators it licenses. The licence number in a casino footer is only a claim until it is looked up there, under the operator's legal name, with the status showing as active.

What the register tells you

Whether the licence is current, which company holds it, and which trading names it covers. A brand missing from the list of trading names is not covered, whatever the footer says.

Which authorities answer complaints

Some regulators run a dispute service that operators must answer to; others only publish rules. Before a deposit, check which of the two you are dealing with - it decides where you can go if a withdrawal stalls.