This is a link to FSW 2014 Document Checklist: http://www.cic.gc.ca/english/pdf/kits/forms/IMM5612E.pdf
Please bear in mind that the system has changed so the above link is not valid anymore, however, the rules for proving work experience still apply (correct me if I am wrong). Here is the relevant part for you:
Work Experience
For the principal applicant:
• Letters of reference from all your employers for the past 10 years.
Letters must be:
• written on company letterhead,
• signed by the responsible officer/supervisor,
• show the company's full address, telephone and fax numbers, e-mail and website addresses,
• stamped with the company's official seal (if applicable).
Letters must include all of the following information:
• the specific period of your employment with the company,
• the positions you have held during the period of employment and the time you spent in each position,
• your main responsibilities and duties in each position,
• your total annual salary plus benefits in each position, and
• the number of hours worked per week in each position.
• Additional supporting documents may include:
• copies of work contracts, and
• copies of pay stubs.
If you and/or your accompanying spouse or common-law partner have engaged in full-time work in Canada,
with appropriate authorization, for a period of at least one year in the past 10 years, provide:
• letter(s) of reference (as per requirements stated above) from your past Canadian employer(s),
• a copy of any T4s if available,
• a copy of the employment authorization.
Important: when submitting your tax information, be sure to black out your Social Insurance Number (SIN) to protect
this personal information.
Note: Self-employed individuals must provide documentation from 3rd party individual(s) indicating the service
provided along with payment details. Self-declared main duties or affidavits are not acceptable evidence of
work experience.