To check for battery drain, you need to measure how much current is flowing when the car is off. With a multimeter you would use one of the amperage scales and insert the meter into the circuit in question. Start with the highest scale and change downward until the reading makes sense. In your case, you would disconnect the battery and use the meter to complete the circuit.

Warning - you must use care around car batteries and have your radio code handy.

To find out what is causing the current that you find, pull the fuses one by one and see how much the battery drain changes. In my case, I found a defective glove box switch. Even with the glove box bulb out, the switch was drawing current.

Do a search to find out what various brick owners think is excessive battery drain.

Fwd Volvos are notorious for short battery life, mine went about 6 months after I disconnected the glove box light switch
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2 8s & 2 7s 633,000 miles total