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You can use several expressions:
set theText to "this is a sample"
set lookingFor to "sample"
offset of lookingFor in theText --> if result is not "0", then it is contained
theText contains lookingFor --> true or false, by default ignores case
lookingFor is in contents of theText --> the same
If you are looking for more options and power, take a look to the find text command of the Satimage osax, or use any command-line tool, such as grep.