It's attaches to the end of the name, whichever part of the name you have. Tóthné, Tóth Lászlóné. I don't know exactly if just Lászlóné is used, but it would be pretty colloquial in any case. It would usually be "László felesége" instead. The Wikipedia has a nice article touching that matter.
Around the middle of the last century it was also common (in America, at least), to attach "Mrs." to the full name of the husband when talking about his wife. So if Sylvia Garner married Jack Robinson, she would sometimes be referred to as "Mrs. Jack Robinson", making clear who she's married to. That naming convention wasn't as popular as the Hungarian counterpart, but you can sometimes hear it in older films and some sitcoms.
But yes, nowadays it's archaic and you'd rather translate this as "Are you the wife of László Tóth?"