Most useful and OTC test is Marquis (I take 5 ml of concentrated sulphuric acid and mixed it with a few drops of formaldehyde (around 40%), always use a fresh solution).
The following color changes can be expected within one minute: (m)amp -> orange changing to brown, Ext -> dark purple / black; opiates -> purple (if you test too much it will look like Ext, just a matter of concentration)
A number of compounds will give a bright yellow color, e.g. 2C-B
A number of compounds will give a brown color because of sulphuric acid burning it
I sometimes use a second test to find out if a secondary amine is present: I´m not sure about the name but it uses sodium ferricyanide (0,25 gram in 14 ml water) and acetaldehyde (2,5 ml), with 8,5 ml water. A strong blue color indicates the presence of a secondary amine.
I know there are a number of other test (LSD -> Ehrlich, ephedrine -> Chen, Cocaine -> Cobalt Thiocyanate, THC -> Duquenois, but I don´t have personal experiences with them. I´m sure more info can be found elsewhere....