Vetting the perfect candidate for a job includes checking with the candidate's previous employer to verify her employment dates, salary and eligibility for rehire. In addition, prospective employers might want to ask a supervisor or manager about your past performance. Inexperienced or hasty recruiters may call before your interview; however, a recruiter most likely will call your previous employers after your interview, as most recruiters consider that the final step in the full life-cycle recruiting process.
Some applicant tracking systems used for online applications ask whether the company can contact your current and previous employers. Unless you're on a clandestine job search in not wanting your current employer to find out that you're looking for another job, you may find yourself in a quandary if you must absolutely complete that field on your application. You probably shouldn't check the "no" box disallowing prospective employers to contact any of your previous employers -- that sends a red flag to recruiters. Address this in your cover letter or resume where you can insert a comment about contacting previous employers.
Recruiters who check your previous employers before your interview may be overly anxious or inexperienced. Recruiters are ultimately responsible for passing on to hiring managers the names of candidates who meet the requisite skills and qualifications for the job. Checking with previous employers might appear to be part of the screening process; however, interviews are a far better method for screening candidates. Further, a recruiter who contacts your previous employers even before he has the chance to interview you may be doing too much too soon, as reference checking is one of the final stages in the recruiting life cycle. It's a waste of time calling an applicant's previous employers before the recruiter has a chance to decide you might be a viable candidate for the job. As a job seeker, you can tell the recruiter that you'd prefer to keep your job search confidential until there's a conditional job offer on the table.
An inexperienced recruiter might believe that calling your previous employers before the interview saves time, when doing so can put you in a difficult position. Recruiters should always ask if they have your permission to contact your previous employers and, thereby, give you the option to say yes or no. Asking a recruiter to hold off on calling previous employers until you've been placed on the short list of candidates the employer is seriously considering is entirely acceptable. Besides, calling previous employers takes up the recruiter's time that would be better spent concentrating on other duties, such as preparing for face-to-face interviews.
The ideal time for a recruiter or hiring manager to contact your current employer or previous employers is after you've completed the substantive components of the selection process, as in a preliminary interview and at least one face-to-face interview. Calling previous employers after an interview also gives your prospective employer the opportunity to ask specific questions based on information the recruiter wouldn't have known to ask before conducting your interview. For example, if you reveal during the interview that you have skills you may have omitted from your resume, the recruiter might want to verify those skills with your previous supervisor. Calling your previous employers after you interview with the company is always the best time for recruiters to conduct work history verifications and reference checks.