You’re nearly there. You do need to disconnect that connector though. Try pushing it together slightly before pressing the lock tab, then pulling it apart. It’ll be tight but it will come apart.
Once you have it apart set your multimeter to the Ohms setting ad connect the leads to the appropriate sockets. Touch the leads to the terminals in the plug that go to the sensor, it doesn’t matter which way round. If it’s an auto-ranging meter it’ll just show a reading if it’s manual ranging you’ll have to switch the range. Start with the highest then work down until the meter goes over-range then switch back one click. You’re looking for around 1.5 kilo ohms for a good sensor.
Does this photo help at all?
Each sensor (front, rear, left & right) can be tested in the same way.
This is a link for how to check for fault codes in the ABS system.
Changing the senor will be a pain, bolts will be rusted and will break off when you try to remove them and the sensor itself will be seized in the hub and reluctant to come out. Make sure that you remove any rust from the sensor mounting hole prior to fitting a new sensor. It needs to be a “sliding” fit and not tight.
ABS sensor and wheel speed sensor is one and the same.