Hi Tony, if the battery has not been changed for five years or more I would replace the battery in any case. It’s cheap and easy to do.
Checking an old battery with a voltmeter will give you an indication of its voltage but not of its ability to keep a system going when power goes off and the battery has a load on it.
If you want a quick test before the meter change simply turn off mains power(assuming there is adjacent fused spur to the control panel). If the system remains operable and the outside bell does not activate you may be ok.
Be aware however that if it doesn’t that particular scantronic panel (it’s actually a 9448, the keypad is the 9427) may sometimes restore to the factory default on power removal and will require re-programming thereafter.
All said and done simply replace the battery now.