There're some rules about on-yomi and kun-yomi.
The reading of 3 or over syllables are all kun-yomi.
e.g. 形(katachi), 魚(sakana), 潔-い(isagiyo-i)...
As for 2 syllables,
EDIT:
all of the ones ending with ん are on-yomi.
安(an), 近(kin), 山(san), 店(ten), 本(hon)...
most of the ones starting with 濁音(voiced consonant) or ら行(ら-column) are on-yomi.
学(gaku), 人(jin), 電(den), 楽(raku), 林(rin), 類(rui)....
all of on-yomi end with one of the seven kana 「イ、ウ、キ、ク、チ、ツ、ン」.
So the others, 赤(aka), 石(ishi), 肩(kata), 谷(tani)…, are all kun-yomi. Of course 皿(sara), 栗(kuri), 春(haru), 彼(kare), 白(shiro) as well...
And I agree with nice gaijin. "on-yomi or kun-yomi" is basically used only in a sort of exam, in my opinion.