This should really just be added to Arkhaic's answer (and I think I saw it in a comment on that answer at one point), but by itself is big enough to be a separate answer and not just a comment: SUNLIGHT
Assuming your party didn't previously avoid sunlight like the plague, it's going to be REALLY suspicious that your teammate no longer welcomes the rays of the Daystar. Like, glaringly obvious. As soon as your character notices, given you've had vampire troubles in the last two towns, your character's mind should jump there immediately.
Other things that haven't been mentioned yet: Buff spells. Some of your friendly team-buffs will no longer work on the vampire, because spells like Enlarge Person require a humanoid target, and he's not humanoid any more.
Healing. Cure spells damage the vampire now. If he takes any damage in a fight, when the cleric goes to heal him and he refuses, possibly recoiling in fear, the red flags should be everywhere.
Healing redux. Again assuming a cleric, but Channel Positive Energy will no longer heal him (when channeled to cure the party), or will harm him (if channeled to damage enemy undead). Noticable, though not as much as the cure spells.
Any private residence you enter. You don't have to break into houses to foil the vampire's "can't enter homes without permission" thing. If someone opens their door for you, but doesn't specifically invite the vampire in, guess who's stuck outside while everyone else walks through the door? What about player housing, where it's pretty much assumed party members can come in, so you don't bother specifically offer invitations to your closest friends?
Running water. No seriously, if you cross a bridge, the vampire can't follow you. Go fishing on a lake during your free time, or rescue some lost kids past the old brook. When he sits out the adventure because he can't cross the brook... I get the feeling I sound like a broken record at this point. I looked at the Pathfinder version of the template, and it doesn't look like crossing over running water still affects vampires. However, immersion in running water still does. It's a bit more meta-gamey, but tipping the boat the party is in should do the trick... fatally, if he's not fast on his Mist Form.
His reaction to getting incapped. This one is a stretch, but most players are ok with going down in a fight, assuming someone with a cure spell is nearby. If the vampire goes down, he automatically goes mist form (and must reach his coffin in two hours or less, giving him a nine mile radius to find it or die forever). This applies even AT zero hp, not negative, when most players are still planning to get one last hit in.
Detect evil. Seriously, he's high-tier undead, he shows up to it. Brightly.
Exceptional DR, resistance to cold and electric, and fast healing. These abilities are normally considered to be obvious when witnessed. If at any point he gets struck for any damage, you will either witness him ignore it entirely, resist it like a complete boss, or suffer a wound that quickly knits like he was somehow stacking five rings of regeneration.
Holy symbols, aka Cleric the third. When the cleric, if there is one, casts most of his spells or uses his channel energy power, he must strongly present his holy symbol. The vampire, if within 5 ft, is forced back. Even if not within 5 ft, he is not unaffected, and he doesn't even get a save until the second consecutive turn the symbol is presented.
This wall of text is getting out of hand, so I'll end it here. TL;DR: Vampirism is a harder condition to hide than leprosy and lycanthropy combined. It would probably be less obvious if flags sprouted from his ears that said "I AM A VAMPIRE I AM GOING TO KILL YOU". You don't have to go out of your way to uncover him. A mirror, a cross, a pizza topping, a pond, a cantrip, THE DAY/NIGHT CYCLE, a cure spell, a closed door, or just getting hit can all give him away.