I don't recall a specific mechanism for Nitrate anion alone.  But you will have to reduce it to Nitrite with Cadmium they spray with a solution of sulfanilic acid.  You should get a 'red' color absorbing at ~540 nm.

I recall using diphenylamine for detection of nitrate, I has been many years since I did this on TLC as a confirmatory test for nitrate. As I remember I sprayed the plate with 5% diphenylamine in ethanol to which was added 10% V/V concentrated  sulfuric acid. Blue spots appeared where there were nitrates. Caution, Diphenylamine is a health hazard and very toxic use the spray in a good fume hood! Although I could not separate them on TLC, ammonium nitrate, sodium nitrate and potassium nitrate all gave blue spots.

inorganic salts does not  move in tlc  you have to use flame photometry or atomic absorption  analysis

Anions are not detectable by atomic absorption/emission.

There used to be a Nitrate Ion Selective Electrode (ISE) but that is not TLC and requires water media.  It was also prone to sample interferences.

Actually some inorganic salts do move on TLC. Potassium Iodide moves near the solvent front on a silica gel TLC plate developed with methanol 0.1% v/v formic acid. I don't know if other salts will migrate on a silica gel TLC plate, but I would suggest giving the methanol 0.1%v/v formic acid eluent a try.

use ninhydrin or h2so4 solution for spary it after delvolpment