Probably the easiest way is to have a friend call your former employer for a reference and act as though they're considering hiring you.
Note: most large companies with (HR departments) will only confirm the period you worked there and your title. They want to avoid the risk of lawsuits.
Having said that if someone contacts your former manager directly they may go beyond the bounds of simply confirming you worked there. A common question often asked of former managers is:
If you had the opportunity to hire her or him again would you? Why or Why not?
Smart managers reiterate the company policy by stating; "I'm only allowed to confirm she/he worked for me and their responsibilities were...etc"
Note: If you listed your former manager as "personal reference" then they're likely to be more candid in answering questions. Ideally no one would list a (personal reference) whom they thought wouldn't give them a "glowing" one.