There's a basic test used to identify bodily secretions at crime scenes. It's not the only test, but it is useful in primary investigations -- unless you spill a lot of commercial flour around.
Your mouth is perpetually full of amylase, which isn't a bad thing. Amylase is an enzyme that attacks starch, breaking it down into simple sugars that the body can digest and store. People who don't eat a lot of sugar, but who do chew their food thoroughly, might notice that over time starchy foods take on a slight sweetness. That's amylase at work.
The salivary glands make salivary amylase, but they don't work alone. A healthy pancreas makes several types of amylase, which is present in multiple other bodily fluids -- particularly the kind found at crime scenes. When testing for saliva at crime scenes, forensic scientists have made use of amylase's ability to break apart starches.
One common preliminary test involves spraying a solution with dissolved iodine attached to a starch. When the starch and the iodine are linked, the solution is one colour. If the starch should happen to be chewed apart by an enzyme, the iodine gets released and reverts to its regular colour.
The problem is, amylase is too useful an enzyme to confine itself to only one animal. It's present in the saliva of other mammals, but it's also found, in great quantities, in flour and yeast. Sprouted grains contain amylase to give the sprouted plant some energy, and yeast uses its amylase to break down the starch around it. When you see dough rising, you're watching yeast use amylase to digest its dinner. As home cooks grew impatient waiting for dough to rise, flour manufacturers started adding amylase to their flours. The amylase does nothing to the flour until water is added, at which point it provides yeast with a banquet and lets it reproduce quickly.
This is all well and good for bakers, but it means that amylase is everywhere, and anyone who wants to make a primary investigation tough for the police just needs to throw around some flour.
Or, more simply, don't lick the crime scene.