Yes, you can get a rough idea of the molecules present in a TLC run using UV light. At short wavelength, compounds with rigid and aromatic structures, such as phenylpropanoids and alkaloids, normally give dark spots on a green background. At long wavelength the molecules give characteristic colours, for example coumarins exhibit blue fluorescence. You can find more information in this book:

Book Thin Layer Chromatography in Phytochemistry

Thin-layer chromatography can only roughly determine whether there are new substances, but not accurately determine the substances. Accurate classification of substances requires testing such as NMR.

To investigate a main compounds which present in plant we must use different instrumental techniques such as LCMS; HPLC; CGMS; TLC; RMN, IRTF... However, a preliminary fractionation is necessary to distinguish the different classes of compounds (flavonoids; alkaloids, terpenoids, etc.).