What happens if you have health insurance through your job, then have to get new health insurance?

If you have health insurance with your employer then quit, how can you get new health insurance with your new employer because aren't they going to see all your pre-existing history from your old insurance company?

If you get join your new employer's health plan, they can't exclude pre-existing conditions if it's a group plan. They have to accept everybody. If you obtain any kind of health insurance within 60 days after leaving your job, by law the insurance company can't deny coverage of pre-existing conditions.