1. First dissolve a sample of the unknown solid in water and measure pH with pH paper. Measure the pH of pure water as a control (this is your neutral).
NaHCO3 will be basic. Salicylic acid will be acidic. If the the solution is neutral, or rather, the pH of the pure water, you have either AgNO3 or KCl.
2. Dissolve a sample of the unknown solid in water.
Test for the presence of Ag+ by adding a solution of NaCl in water to the unknown solution. If a white ppt forms, you have made AgCl, and this is positive for AgNO3:
AgNO3(aq) + NaCl -> AgCl(s) + NaNO3(aq)
If no ppt, then you have KCl. To confirm, test the sample for chloride by adding an aqueous solution of AgNO3:
KCl(aq) + AgNO3(aq) -> AgCl(s) + KNO3(aq)
To further confirm, perform a flame test with the unknown. Dissolve a sample in water, dip a Q-tip in the solution, place into the flame of a bunsen burner. K+ ion will produce a pale violet flame.
Hope this helps!