You can easily check the headset by (working upside down) removing the stem and spacers, then gently lifting the fork to check the complement of balls in each bearing. Use a small screwdriver or icepick to gently move the balls around and get a sense of how many are missing by the gaps. If it's truly only short one or two, ignore that and ride. Buy a pack of balls, and schedule the headset overhaul for when it's convenient --- there no rush. OTOH if you're missing a few (gap of 3/4" or so) you should do the overhaul sooner rather than later.
Very rough guideline is that you can have a gap, of up to 50° of arc with little or no loss of function.
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