How can I check if a string is a float?

You can use the parseFloat function.

If the value passed begins with what looks like a float, the function returns the value converted to a float, otherwise it will return NaN.

Something like:

function beginsWithFloat(val) {
  val = parseFloat(val);
  return ! isNaN(val);
}
console.log(beginsWithFloat("blabla")); // shows false
console.log(beginsWithFloat("123blabla")); // shows true

Like this:

if (!isNaN(value) && value.toString().indexOf('.') != -1)
{
    alert('this is a numeric value and I\'m sure it is a float.');
}​