Check whether a cell contains a substring

Try using this:

=ISNUMBER(SEARCH("Some Text", A3))

This will return TRUE if cell A3 contains Some Text.

This formula seems more intuitive to me:

=SUBSTITUTE(A1,"SomeText","") <> A1

this returns TRUE if "SomeText" is contained within A1.

The IsNumber/Search and IsError/Find formulas mentioned in the other answers certainly do work, but I always find myself needing to look at the help or experimenting in Excel too often with those ones.

The following formula determines if the text "CHECK" appears in cell C10. If it does not, the result is blank. If it does, the result is the work "CHECK".

=IF(ISERROR(FIND("CHECK",C10,1)),"","CHECK")

For those who would like to do this using a single function inside the IF statement, I use

=IF(COUNTIF(A1,"*TEXT*"),TrueValue,FalseValue)

to see if the substring TEXT is in cell A1

[NOTE: TEXT needs to have asterisks around it]