Following are step-by-step instructions for field testing blotter for LSD. This method would test positive in the presence of most (any?) indole substances, thus is not specific to LSD, but can determine whether the blotter is simply blank.

1. Extract a section of the blotter with a minimal amount of ethanol. (enough to leave you with a few drops of liquid)
2. Put a drop of the extraction on a 4-(dimethylamino)-benzaldehyde impregnated piece of filter (or chromatographical) paper.
3. Allow to dry.
4. Put one drop of concentrated hydrochloric acid (muriatic acid will work) on one side of the paper. A colored front should migrate over the paper (concentrating the color).
5. A purple colored ring will appear (changing to blue 30-60 minutes later) if alfa or beta unsubstituted indoles are present (the whole paper will color if large amounts are present). This purple / blue coloring signifies the presence of LSD.

According to the source, this simple field test is able to detect 7.5 micrograms (ug) of lsd without a problem (typical doses were listed as 75-250 micrograms; you should get that lucky or unlucky for that matter)