If you just look at the console, Firefox will tell you whether your asm.js validated or not. In the case of the script pasted above, I get:
TypeError: asm.js type error: non-expression-statement call must be coerced test.html:23
which suggests not. Line 23 in this case is:
return +sqrt(square(x) + square(y));
If I replace that line with this:
return +sqrt(+square(x) + +square(y));
then the error console says:
Error: successfully compiled asm.js code (total compilation time 0ms)
which means it all worked fine (and the "Error" there is just a bogus quirk of how the JS engine reports success in this case; I filed https://bugzilla.mozilla.org/show_bug.cgi?id=950527 to get that fixed).