QB Issue Resolution:

There is no Conversion. They are both separate Transaction Types that both exist.

Open the Estimate and use History (on the ribbon, or Ctl H) to see invoices linked to it.

On Invoices, you can also use History, and a progress invoice has the little Progress Chart Icon at the top, right of center, in the ribbon, as well.

Resolution for Issue 'How do I know if an estimate has been converted to an invoice other than obviously looking for an invoice that matches the estimate?' available: Yes (Solved).
Source: Intuit Community forum.

How do I know if an estimate has been converted to an invoice other than obviously looking for an invoice that matches the estimate?: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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