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When you encounter a lamp with an unknown history, you need to use judgement. With the proper safety equipment (full face shield, protection of neck area and hands/wrists, etc.), carefully examine the cold lamp. Is the bulb severely darkened or miscolored? Any sign of unusual heating of the contacts, electrodes and seals? Any vitrification (hazy or crystalline appearance) of the quartz? Are the cathode and anode surfaces in good condition, without severe pitting, erosion, or "mushroom" growths? Does the lamp strike reliably? Is the plasma stable or does it tend to wander and flicker? How's the image on the screen (no flicker, good uniformity,meets SMPTE standard)?

If the lamp looks good in all respects, continue to use it, but be more than critical of any change in performance. And of course, keep a good logbook of maintenance, so you can keep track of the performance.