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The service tag is usually on a sticker on the computer
For a laptop, turn it over on a flat surface to see the bottom
For a desktop, look on the side or top of the tower
Dell also has further instructions available.
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The serial number is usually engraved in small text on the bottom of the computer near regulatory text.
For a MacBook, turn it over on a flat surface to see the bottom
For an iMac, lay it down on a flat surface to see the bottom of the foot/base
Apple also has further instructions available.
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