Why does the Torah use the expression "Vayichbad Leiv Par'oh" here?
Hadar Zekenim: After the first five Makos, Par'oh did not willingly detain Yisrael. Hashem forced him in order to show His capability.
Why specifically here did Par'oh check out whether any of the animals of Yisrael had died and not earlier?
Because whereas there was no way of checking regarding the previous plagues, it was possible to check the animals to see whether any of them had died. 1
See also Oznayim la'Torah.
Why does it say that nothing died from Yisrael's flock "Ad Echad"?