My future spouse seems to like spending big. How do I judge whether this is a problem before we marry?

Look at the pattern, not single purchases; what is worrying is spending that exceeds income or is funded by debt. Notice a few things: do they save part of their salary, do they often borrow for lifestyle, and how do they react when they cannot afford something. Ask openly, ‘Usually how much of your salary do you save each month?’ Share your own habits too, so the conversation goes both ways. Allah praises those who, “when they spend, are neither extravagant nor miserly, but hold a balance between the two” (Surah al-Furqan 25:67). If they are open to change, that is a good sign. If they deny it or get angry, take it seriously before making a decision. See criteria for choosing a spouse.