From the manual on system expressions:
Find the first index within src that text occurs, else returns -1.
So, you can use find(textbox.text, "sit") and if it comes up with anything other than -1, the word sit is in there somewhere. Unfortunately, it would also find any words containing "sit", such as situation. If you wanted to make sure it was a specific word you could go with:
for "loop" 0 to tokencount(textbox.text, " ")
-Compare to values: tokenat(textbox.text,loopindex("loop")," ") = "sit"
That would break the string up by spaces (i.e. words, if the person isn't a bad typist like I am) and check each piece individually.