They ask for a letter of reference from the employer, with very specific instructions on what is to be included in the letter, as well as paystubs (salary slips), and whatever other documents you can provide as proof (for example, if the experience is in canada you might submit T4 statements and an ROE is applicable). They also may or may not contact the employer directly, it is up to the agent. Contact is likely by phone, email and/or letter.
Here are the instructions (as of May 22nd) from the document checklist:
"You must provide proof of each of your current and previous work experiences.
Your evidence should include a reference letter from your employer and previous pay stubs, where available.
The reference letter is an official document printed on company letterhead and must include your name, the company’s contact information (address, telephone number and e-mail address), the signature of your immediate supervisor or personnel officer at the company and a business card of the person signing the letter. The letter should indicate all positions held while employed at the company and must include the following details: job title, duties/responsibilities, job status (if current job), the dates you worked for the company, the number of work hours per week and your annual salary plus benefits."