There's a tell-tale with Seiko 6139s that I presume is common to other models too, though it's hard to check unless you have photos of the service and they've bothered to picture the back of the dial - the first 2 digits of the case serial number are printed on the back of the dial, eg this Pogue:
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I had it serviced and explicitly asked the watchmaker (who hadn't heard of this previously, I think) to take a pic of the dial back:
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