Identify KNO3 and KH2PO4 at home
In the absence of other chemicals you can utilize the fact that nitrates(V) are much stronger oxidizing agents than phosphates(V).
You can soak a piece of newspaper in a saturated solution of the salt, let it dry well in a ventilated place and try to ignite it.

If the salt was $\ce{KNO3}$, it will burn vigorously and fast (that's why kids are often using paper soaked in sodium nitrate as primitive fuse for home-made firecrackers and rockets).

$\ce{KH2PO4}$ doesn't promote combustion of paper, so it will burn normally or even slower.