Generically, if a frame exists it will be a table instead of nil.
if SomeModsFrame1 then
-- frame exists
end
Lots of times when I forget the name of a frame or function, I'll do /script message(IsAddOnLoaded) and it will say table or function or nothing for nil. (well, specifically I have in my startup: function print(v1) DEFAULT_CHAT_FRAME:AddMessage(v1 or "nil") end, and use print instead of message.