Senzu beans are tough to find... I can tell you a good place to test using them is esf_riverside, heres what to do:
-Search the RIVER for any pouch looking items.
-Once u run over it, your Bean Icon will jump to 3 (Usually 3)
-Then you switch the the attack that is AFTER the melee attack (It looks like a clump of dirt that is sweating, lol...)
-Then you hit your fire button, and he eats a bean! Full heath and Ki! Works great if ya have em! Only need one really...
Hope this helps mate.