I worked for a company about 15 yrs ago and I need to know how I would check to see if I still have my 401k plan

First you contact the old company and then the Human Rescoures dept and tell them you left the company around 15 years ago and forgot about your 401K and you need to find out who the contact person would be to check into it, if you invested in it or even if they did for you (although most companies if leave before a year is up keep the money) , you will some vested amount in it, just because you left does not mean it does not still exist. The money would still be there however it would no longer have had employer matches or anything. Once you find out if it still exists or not get the information and if you have an IRA contact them and allow your IRA broker to have it rolled over for you. Good Luck

Well most companies have their 401K managed by a investment firm, you should be getting yearly reports of the money.
But contacting that company and finding out where or who is investing the money is the start. Again all the money you put in, should be yours normally, the matching part is often loss if you did not work for them long enough.
I was in a union over 30 years ago, never moved out my small retirement I still get a notice every year from them.

When you terminate with a company, they are required to file a form annually with Social Security that indicates you have a pending benefit. When you file to collect Social Security they will inform you of any pending benefits you have. If you contact them before retirement you may be able to get the info.