I've never tried Google calculator, but it doesn't seem like it should be too difficult to detect at least fairly simple math expressions.

Something like:

# Assuming expression is in $string
my $num  = '-?(\.\d+|\d+(\.\d+)?)';
my $ops  = '[-+*/]';
if ($string =~ /^(${num}${ops})+${num})$/) {
    # It's a simple math calculation
} else {
    # It's something else
}
[download]

Now that's pretty basic, but it should parse expressions of the form number op number [op number ...] for numbers containing an optional leading "-", and at least one digit (with possible decimal point), and for any of the basic operations {+, -, *, /}.

For anything more complicated than that, you'll have to ask someone with a more intimate knowledge of Google calculator than I.