The command (or method in M2 speak) you're looking for is isField
rather than IsField
.
i1 : R = ZZ[x];
i2 : I = ideal(7*x^11+4*x^5-23*x^4+x-27);
o2 : Ideal of R
i3 : S = R/I;
i4 : isField S
o4 = false
i5 : isField (ZZ/2)
o5 = true
But there is (or at least was as of February 2014) a major caveat about using this internal M2 method for testing whether a quotient ring is a field (alternately testing the maximality of the ideal).
More generally, the applicable rules from the Macaulay2 style guide for naming methods and variables are
Names representing methods must be verbs and written in mixed case
starting with lower case.
The prefix "is" should be used for Boolean variables and methods.