Diagnostic tests: 
Thorium is usually detected by radioactivity (due, in part, to the daughter elements such as radium). Uranium is also naturally radioactive; however, many uranium minerals are generally yellow or green and fluorescent, so if not, then suspect Th. Any radioactive mineral with a negative reaction to uranium tests is probably thorium.

Thorium is usually associated with the rare earth elements and use complicated schemes to identify the element. Basically, the rare earths and thorium are precipitated as hydroxides from acid solutions. The precipitate is dissolved in HCl and precipitated as insoluble oxalates, washed, filtered and ignited to the oxides. The oxide is dissolved in H₂SO₄ and ignited to dryness. Dissolve the sulphates in water and precipitate Th using boiling sodium thiosulfate solution.  Repeat the thiosulfate step if necessary to eliminate excess Ce, Zr, and Hf.

Infusible thorium minerals glow with exaggerated incandescence in the flame test (along with Sr, Mg, Zr, Zn, Ca - the "lime light effect").