Is social security number required or checked by retirement plan companies from work?

I am very curious to know if whenever you apply at work for your retirement plan, does your ssn checked or verified in order for them to estanlish the plan? I need to know bc this had been spinning in my head and i do not want to ask any employer for it, bc its not me with the problem, i do have my ssn. Does anybody know any of this info?

Yes, it is easy to check and they do.