How can I go about getting information telling me if I have a401k plan

You need to contact your old employers or their successors. You should be getting statements, at least annually. Also, any retirements benefits you might have will be reported to the SSA who will inform you when you apply for Social Security benefits.

Do you mean in your question how do you find one that you had in the past to see if it is still open ? A 401K is not something that just opens by itself, you would have signed paper work and even paid into.

So they also should have been mailing you at least annual reports on your money, and how it is invested.

But if you have no idea you will have to contact all of your past employers and ask if they have any record of it.