If the code is implemented within Customer Events, you will not be able to see GTM code and dataLayer as it runs within a sandboxed environment, and you will not be able to see events within the GTM debugger on checkout pages. There is the old Tag Assistant, but it is not very reliable. The options to validate are:
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To generate console.logs to test the events
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Try to see events in Real Time report in Google Analytics 4 (events in real time sometimes do not appear quickly enough)
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Try the code in a test or dev store with similar setup to the one you are trying to verify.