Maybe you two find yourselves discussing a long-term future together (or you've *casually* hinted about your dream engagement ring). There could be other explanations to these behaviors, but he might just propose soon if...
He suddenly goes from big spender to big saver.
Remember that new TV he had been saving up for? How about that watch he couldn't wait to buy for the longest time? If out of the blue he loses interest in such an investment, that might just translate to the next step of your relationship. Engagements and weddings can cost a pretty penny, so if he's being strict with his budget, this could definitely be a sign he's about to slip a rock on your finger.
One of your rings just happens to disappear from your jewelry box.
If your friend, roommate or sister didn't borrow it, it's a possibility he's "investigating" your ring size. And don't be surprised if he gains a sudden knowledge about diamond cuts or the latest engagement ring trends, too.
Your family and friends act overly excited whenever you call.
You've noticed they're acting slightly more bizarre than the usual. In every conversation you sense some anticipation in their voices, or maybe they've started dragging you into jewelry stores while you're out shopping. Either sign suggests he's asked your family for their permission.
He takes the initiative to make BIG plans for the two for you.
Suddenly, he's just booked a beach vacay for two around Valentine's Day. Or maybe he decided to schedule a dinner reservation at the restaurant where you had your first date (bonus points if it's on the same day, too). Just go along with it and act surprised, even if you think it's coming.
He not-so-subtly asks you how he should propose.
Don't get blindsided by this one. And when he's getting down on one knee with a velvet jewelry box in hand, we hope you catch his drift.