As a conversational tag for a rhetorical question, the expression I've heard is "my a omwovyn". You can add this to the end of a simple question, and thereby avoid the complexities of conditional clauses, e.g.A question related to the medhes/leverel 'reporting' type verbs that can introduce a reported, dependent clause: how does one say : ' I wonder if ...' ' I wonder what..' etc ?
"Py par cath yu honna, my a omwovyn?" (What sort of cat is that, I wonder?)
"A vyn Jowan dos dhe'n dywotty genyn-ny haneth, my a omwovyn?" (Will Jowan come to the pub with us tonight, I wonder?
As Nance (1955) points out, though, 'wonder' can also be used in other contexts:
puzzlement/surprise:"Marth a'm bus anodho"/"Marth yu genef anodho." (I wonder at him)
uncertainty: "Dowt dhym yma na vova gwyr." (I wonder whether it's true). Note that the use of the subjunctive suggests that I think it's unlikely to be true; the English would be more accurate perhaps if it read '...if it might be true'
confirming a prediction: "Nyns yu marth kyn fo ..." (No wonder that it is ...)
Again, the subjunctive suggests it might not be so; for a more definite statement, I'd use:
"Nyns yu marth bos ..."
Williams (2006) adds:
ignorance: "Hy a vyn govyn orty hy honen pleth eson ny." (She'll be wondering where we are)literally, 'she'll be asking herself ...'