It's hard to tell because God doesn't speak audibly to most people. Many people make the pitfall of judging by what they feel God is telling them. Don't go by what you feel...when did God ever speak to any of the prophets just by a random feeling? No...if you're romantically interested in someone, I guarantee you'll feel something. It's called being smitten. It's not necessarily from the Holy Spirit. I have a strong feeling right now called being hungry...is that from the Holy Spirit? Anyway, yeah, don't go by feelings.
Instead, read the Bible. It's called God's Word for a reason; He speaks words through it. The Bible says lots of things about marriage; does this relationship fit into what the Bible says? If you don't know the girl well enough to know, then it's time to wait until you know her better.