A letter of reference is the ideal way to prove your employment history. But suppose there is no way of obtaining one. In that case, you could try to confirm everything by submitting other documents like payslips, letters of appointment, employment offers, letters of promotion, documents confirming the work you've done, etc. It is good to provide a signed letter detailing the reason you are unable to secure an immigration reference letter.
A perfect immigration reference letter doesn't guarantee success regarding your Canadian Permanent Resident Visa application. The final decision lies with the visa officer dealing with your application. Ensure that the employment record you submit is complete, correct, and true. Amplify your credibility by submitting validated information.