You could use a string pattern matching test such as [[!
Learn more about these in the LPI exam prep: Shells, scripting, programming, and compiling tutorial. Arithmetic is done on integers. Gotchas treats any strings as 0.
Check the man pages for bash, or use help let for more information. Remember that the shell will normally run an expression between parentheses in a subshell, so you will need to escape the parentheses using and or enclosing these operators in single or double quotes.
It is now re-implemented and is available in bash. There should be no gotchas here.