Presumptive test for modafinil?
Complex structure

Modafinil is a pretty complex molecule and judging by its structure (see below) I think it will be extremely difficult to find a single reagent test that will identify it.



Post-consumption identification

Interestingly it is much easier to find after consumption, either by the strong sulfur smell of your urine or through chromatography techniques (also here).

'Sketchy' tests

There are apparently some simple tests you can try like dissolving it in hot water and vinegar (which should happen without formation of gas), but these tests are far from fool-proof and might even cause you to think something is a fake while it is not. 

Combination of physical/chemical tests as a 'solution'

The best thing to do if you really want 'kitchen-level' tests is to test for a whole range of chemical/physical properties. For example this reference has some information on the pH, water solubility and melting point which are fairly easy to test. Passing the tests will certainly not guarantee anything, but at least it will take away a significant part of the doubts that the product is fake.

Something to consider when designing these tests is that there will always be fillers (e.g. lactose monohydrate, microcrystalline cellulose, pregelatinized starch, croscarmellose sodium, povidone, and magnesium stearate) in the sample so you have to find ways to correct for that.