How do I find out if I have a 401k plan?

You should know... you would have had to been an active participant... meaning you sign papers, decide how much you will be putting in... and will have contributed directly from your paycheck automatically on a regular basis.

If you didn't... then you don't have one.

Do you mean from your current employer or a past employer?
Do you mean a balance or the ability to contribute?

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I can tell you that when you leave a company, they are required to report any retirement benefits due you to the SSA. When you file for Social Security benefits, they will inform you of any corporate benefits due you.

Past employers

Then contact your employers human resource department.

You should still have known... they send quarterly statements... and I would never forget something like that. Nodoby has a 401K they never knew about. Forgotten about yes. Never knew about? Not a chance.

I have to agree if you ever had one , you would know, you may forgot how to contact them, but you would have paid money into the plan for the years you worked at a company.

So did you ever have a plan you paid into?

Yes , with my past employee

They would still be sending you quarterly statements. I have one with an employer I haven't been with for 19 years that I get statements from. I haven't rolled it because it has excellent options the one I have now doesn't have.

Yes , with my past employee

Since you are not the person who originally asked, did you read the answers given that person? The question was answered, what more did you want to know?