but only the right pair should have dial tone on
You could always stick a voltmeter across them if your unsure.
B wire should have around -50 v DC on it to earth
A wire should have 0v on it to earth
Conditions on the line may affect the voltages though so don't expect it to be exactly that.
Failing that leave a telephone connected off the hook and touch the ends on the A/B terminal when you get dial one you got the right pair.
Once you've found the pair with dialtone you can dial 17070 on the phone (or 147017070 if your line is set to withhold number) it should announce your phonenumber then press option 1 for a ringback test and verify your phone rings ok.
---------- Post added at 18:34 ---------- Previous post was at 18:24 ----------
EDIT: just looked at your pic.
The cables coming in from outside should not connect to the front plate at all!
On the other side of the piece with the screwdown terminals should be a socket, that's the test socket you need to be plugged into when doing testing.
Who wired your phone socket up?