Use an ohmmeter, or continuity tester. Attach one lead to the curvy part. Touch the other lead to each of the terminals. The one that shows a reading, is the solder terminal for the tap. Now remove the lead from the curvy part. Touch it or clip it to the knurled ring on the jack touch the other lead to the other terminals that did not show a reading before. The one that now gives a reading, is ground.
It appears from the , terminal one is ground, two is the tip, and three is the make/break, which you won't use.
Good suggestion about using the meter.
However I do not know what picture you or some of the other contributors are looking at, but clearly in the OPs picture:
1 = Tip
2 = switched terminal - not to be used in this application
3 = ground.
If you look at the picture you will see that:
Terminal 3 is in contact with the tubular section of the assembly. It is sandwiched between the portion of the main body that is peened over to hold the assembly together and the top insulating disk.
Terminal 2 and the switched contact is sandwiched between the top and second insulating disk in the assembly.
Terminal 1 and the tip contact are below/behind the second insulating disk. There is another insulator between the contact and the knurled ring, it is not clearly visible in the picture.
I know how these jacks assembled. I can clearly see how the one in the OPs picture is assembled.
I don't understand how people in this thread are giving such in correct advice. Look at and study the picture the OP posted in post number one. Thankfully we are not dealing with a stereo jack in this case, heads would be exploding all over the place, mine included, but for different reasons.
I will apologize in advance if I have offended anyone with this post, but I am so tired of seeing scenarios like this happen in these forums. Someone gives correct information which answers the OPs question and then a number of arm chair experts with little to no experience/knowledge of the subject matter of the question chime in with their two cents and muddy up everything. If you are not absolutely certain you know what you are talking about don't offer your advice. Don't guess at the answer at the risk of confusing the person looking for help. If you are just out to up your post count, are board and need something to do or just love to type, go to one of the non-technical sub-forums in the Tavern section and strike up a conversation there.
/rant off