By showing the history of the practice: incorporation records, historical tax returns, financial statements and the sale documents. Lawful source is demonstrated through records of how the wealth was earned over time, not just the final cheque.
The record of a career is the evidence
Hypothetical example: a Quispamsis dentist opened her practice thirty years ago with a bank loan since repaid. The USCIS requirement is evidence that the capital was obtained through lawful means, traced along its path. For a professional practice built over decades, that means the original incorporation and loan documents if they exist, tax returns for as many years as reasonably available, financial statements, the practice appraisal and sale agreement, the closing statement and the bank records showing the proceeds arriving and moving toward the investment.
Gaps in very old records should be explained honestly with whatever secondary evidence exists, such as accountant letters and provincial registry filings. If the proceeds pass through a professional corporation, the corporate resolutions and the dividend or wind-up documents must show how the funds reached her personally. Any records in French require certified English translations for USCIS.