How could I find out if someone has U.S. citizenship without being an employer? They have a SSN but I'm not sure if that means they are a citizen. Could they also be here on a Visa? I am not in a position to ask them out right but I need to know.

Why do you need to know ?

Friend is looking at marrying this person and they have recently talked about going to their home country for a ceremony. They are worried it could be they want them to stay there. Or they only want to marry them for citizenship.

Your friend should ask this person directly, he/she has the right to know the other party's legal status, etc. If the relationship is genuine, nothing to worry about. However, it's always a risk when you marry an alien and bring him/her to US, plus immigration procedures are costly and time consuming.

True and it's nothing they wish to be involved in. they have asked and are met with 'you should never question me' - the person was supposed to have obtained citizenship about a year or a two ago. Just want to find out for sure. And if a person has a SSN are they a citizen or can they still get a SSN with just a visa?

SSN are issued to all who has obligation to pay taxes, this covers:
US citizen, permanent residents, aliens with employment-based visas, and those who entered US on some exchange programs.

Only 2 things that are reserved for US citizens: the right to vote and serve as a jury.

When someone claims he/she is a US resident, it does not guaranteed that he/she is a US citizen. Tell your friend DO NOT jump in to this relationship so soon and without knowing the whole truth about this person's record. Not all US citizens are eligible to petition alien spouse.

Get a private investigator and run background check on this person. If there is one or more of the following, just say NO.

Income below 125% of poverty guidelines.
Criminal records.
Domestic violence, sex offender.
DUI.
Failed to pay tax in the last 3 yrs.
Owe child support.
Owe any restitution, or government loan, etc.
Have petitioned more than two alien spouses.
etc.