The simple answer is you can not. Normally when you get cells from a repository or a company you might be more assured but you can never be 100 % confident a new line is non-contaminated. This is why it is best to have a quarantine incubator or a way to grow new cells when they arrive, away from your other cells so that you can test for a problem before you freeze back or introduce your new untested cells into your working cell culture room or incubator.