First one translates into:"Are you sure you want to marry her?"
"Are you sure your company stock price will go up tomorrow?"
This is not exactly what the English original asks; the backward translation of this sentence, Flaminius-sensei, into English would be Is it true that you'd like to marry her?First one translates into:
This one is okay because there's no way for anybody to 決め-rize the stock prices, can them?while the latter
That's why Flaminius-sensei always calls for context. But, as awkward as it may seem, Flaminius-sensei translated your sentence ad hoc and you agreed with her translation as if you both had a previous agenda which I was, of course, not aware of."Xさんとけっこんしたいのは決めた？" sounds like I'm asking whether he makes a decision which I don't think is a correct translation.
"Are you sure some_decision_here" implies the person already makes up his mind. It's I who questioning the wise-ness of his decision.
That's exactly what I said: no kime. And, now, one more time: I am NOT sukebei.The second one has nothing to do with 決め. I'm seeking a confirmation for a prediction. For example, the person may have insider information that supports his prediction: "We will announce winning a big contract latter today. The stock will surely jump after the announcement".
My agendum was to demonstrate how the two sentences can be translated with は確かだ. It awkwardly coincided with what chu_bun wanted to know.But, as awkward as it may seem, Flaminius-sensei translated your sentence ad hoc and you agreed with
herhis translation as if you both had a previous agenda which I was, of course, not aware of.
I doubt if anyone ever called you sukebe.And, now, one more time: I am NOT sukebei.