Enter find mode
put >150 into the number field (won't work for text fields)
Perform the find.
This can also be done with a script.
Ok. The whole field is numbers so that will work just fine
But it's the type specified in manage | database | fields that makes the difference not the data you enter into the field. You can put numbers into a text field, but when you do,
"2" > "1000"
becomes a true statement because Filemaker treats the data as though it were a word (where "x" is greather than "abracadabra") instead of like a number.