Does the nth char equal the nth from last char?
JavaScript (ES6), 26 bytes

s=&gt;n=&gt;s[n]==s.substr(~n,1)


Alternatively:

s=&gt;n=&gt;s[n]==s.slice(~n)[0]


This one almost works, but fails when n == 0 (because s.slice(-1,0) == ""):

s=&gt;n=&gt;s[n]==s.slice(~n,-n)


Another 26-byte solution that @RickHitchcock pointed out:

s=&gt;n=&gt;s[n]==s[s.length+~n]