Short answer
A barcode number is valid when its last digit, the check digit, matches the one calculated from the other digits. EAN-13 has 13 digits and UPC-A has 12. Enter the number into the free GTIN checker on this site and it tells you whether it is valid and what the last digit should be.
Free tools for this
GTIN, EAN and UPC
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Try it now: Barcode Generator and GTIN Checker → · Merchant Center Readiness Check →
A valid number is not the same as a real one
A number can pass the check and still not belong to your product. Anyone can calculate a valid check digit for a number they made up.
Google Shopping and marketplaces compare the number to the GS1 registry, the body that issues them. A number you generated yourself draws a barcode that scans, and still fails that comparison.
Where Indian numbers come from
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What to do if a product has no barcode
Do not invent one. In Google Merchant Center, mark the product as having no identifier and give a brand and part number instead.
After a store migration, the barcode field is often full of junk that looks real, such as timestamps or internal IDs. It fails the check, and because the field is filled, the fallback never switches on. Sort the column and look at it.