You could use a regexp:

var regex  = /^d+(?:.d{0,2})$/;

var numStr = 123.20;

if (regex.test(numStr))

    alert(Number is valid);

If you're not looking to be as strict with the decimal places you might find it easier to use the unary (+) operator to cast to a number to check it's validity:

var numStr = 123.20;

var numNum = +numStr; // gives 123.20

If the number string is invalid, it will return NaN (Not a Number), something you can test for easily:

var numStr = ab123c;

var numNum = +numStr;

if (isNaN(numNum))

    alert(numNum is not a number);

It will, of course, allow a user to add more decimal places but you can chop any extra off using number.toFixed(2) to round to 2 decimal places. parseFloat is much less strict with input and will pluck the first number it can find out of a string, as long as that string starts with a number, eg. parseFloat(123abc) would yield 123.