How to know if Mosque is Shia or Sunni?

I was researching to find some mosques in an area/state and came across one that provides Nikah services. When reading their document in terms of a Marriage contract, it had the word Shi'a in it and it made me wonder if the marriage contract/process is not a sunni process. how does one know if the marriage contract AND the mosque is of a sunni or shia sect? I quote from their marriage contract document: "...Shi’a Ithna-Asheri (Ja’fari) laws of Islam..." It seems difficult to find a mosque who can perform a straight forward sunni Nikah. Does anyone know in the state of Washington or Portland if there sunni mosques who can perform interfaith marriages?