[disillusion]

Helvetica has a few very easy tells; look at the 'e' and 'c', the 'F' and 'E' and the 'A' and 'Y' in these examples, and you'll get 20/20. Oh, and the 'G' and 'R' ofcourse.

[frabert]

Also the lower case 't' is diagonally cut in Arial, straight in Helvetica

[icebraining]

The 's' too - the ends stop in horizontal lines, in Arial they're diagonal.