Actually, looking at the picture closer, it depends on how the terminals are wired.
If the 2nd line goes on 5, then you would wire into green/gray (treating gray like it is red). If not, you will hit yellow/black on terminal 3.
If the line is active on terminal 5, you should be able to hit a test phone on that jack. Otherwise, you'd have to have test leads or a 2-line phone if it is on yellow/black on terminal 3 in order to hear dial tone.
My assumption is that block is set for 4 pairs of wires to hit the box, and it depends on what is activated.
You can make a ghetto test lead by simply taking a silver satin core, cutting off an end, and stripping back red/green to toss on the posts to test for dial tone.