First thing to check is if it is really a joint that is leaking, or that a leak higher up is accumulating on that lip at the joint, and making it look like it is there. If you can physically see water coming out of the joint, then you'll probably need to replace that section of pipe. Dry it off and then have someone flush the toilet while you watch. If it is say a poor seal between the toilet and the flange, repair that or the floor will rot and give you other problems. Depending on where it is and access, you can buy a special tool to ream out a piece of pipe from a fitting so you can install a new section. Depending on what's involved, and since you may never need the tool again, it may be cheaper to just cut the pipe and rebuild, if it is truely leaking rather than the flange/toilet seal leaking.
Note, it wouldn't be the first time that someone forgot the cement! Can you see any purple from the cleaner/primer? (note, not every place requires the purple primer - there is a clear one, but the purple at least shows it was prepped for the cement).