So yes, here it is...
#1. You are on the date and you notice your date is carrying surgical gloves.
#2. He or she invites you over and then goes down into the dark, gloomy basement - and is gone for unusually long periods of time. (Tip: Don't go down there!)
#3. They are unusually clean - like beyond OCD clean, I am talking like Hannibal Lecter clean. Call it one of those weird hallmarks of serial killers, they are unusually fastidious. They make "Mr Clean" look dirty and normal.
#4. The person you are dating is "too good to be true", lives alone, owns property and has lots of disposable income - which means they have lots of land to bury the bodies, lots of money to buy weapons/tasers/pepperspray/etc... and yet they're wasting their time with you??? Something is wrong!
#5. They drive a plain white van with blacked out windows. That is probable cause right there.
HAPPY HALLOWEEN LADIES AND GENTS!