You did not mention which instrument you have. If it is an Agilent 1100 or 1200, use the Diagnosis features for the detector to measure the various spectral parameters. The results will tell you if you are within the specs set by Agilent.
The safest approach is to replace at regular intervals, even if the equipment is working properly. But this is almost never the cheapest approach. You can use the Early Maintenance Feedback (EMF) feature in the Agilent systems to keep track of usage, but as others have noted, this may not be helpful for lamps, since their lifetimes are not predictable.
The only other option would be to continue monitoring lamp performance on a more regular basis. Once a month, or even more often if necessary. If you see a steady decline of a short period, or if you see a further decrease of, say, 10 - 15%, then maybe it is time to replace the lamp now. If the decline is very slow, then maybe you have some more time, but I would agree with everyone else that you should buy a new lamp soon. The problem is that once the lamp really starts to fail, it will go quickly. So you can try catch it when it just starts to fail, but you do increase your risk of a complete failure.