How do i check for letters?

if <(answer) has [letters]> then
say [NO!]
end

Do you mean checking for a specific letter or checking for letters in general, as -Valtren- below showed?
To check for a specific letter, use this block:

<[apple] contains [a] ? :: operators>

To check for letters in general, use this:

((answer) / (1)) = (answer) //returns true if it's a number; false if it's a string

the following should work:
first, make a list that contains all the units

then check whether the first letter of the text is contained in a list. it will be likely a number if it does.

this returns true, but if you put in a letter it would return false