Scratch

FontExists: t1 
    | t2 |
    t2 _ UnicodePlugin getFontList.
    (t2 includes: t1)
        ifTrue: [^ true].
    ^ false

(t1 is a String argument.)

Basically what this code does is call getFontList to t2, making t2 an array containing string names of every font on the computer.  Then, if t1 is part of that array, it reports true.  Otherwise, false.

The Problem:

It will always return false.  And, interestingly, if you replace the t1 in (t2 includes: t1) with 'Arial', it works.  Same if put this in the code part in the workspace, define t1 as a temporary variable instead, and set it to 'Arial'.  But as it is above, it doesn't work.

I tested this extensively and can't figure it out.  If someone can fix this block, I'll share it at the Block Library with credit to me as creator and credit to whoever discovered the solution as an improvement.  Please help!