After waiting over night the blood may no longer look like blood. It will dry and sink into whatever it landed on.

Here's a cheap trick I can recommend. Take along a spray bottle full of Hydrogen peroxide and spray it on any spots that might be blood. If it's blood the peroxide will foam up, if not it won't. Before starting out, go and find some definate blood from the day before and spray some on it so you can get a feel for what it's gonna look like.
You can use any old spray bottle (I'm using an old washed out febreeze bottle) and peroxide is cheaper than bottled water. I think I paid .88 cents for ~32 ounces.
It works great and will help you to know if your looking at blood or just a brown spot.