Parse the number into a string, split by a period and check whether the second item's length is 2:

function isPrecise(num){
  return String(num).split(".")[1]?.length == 2;
}

console.log(isPrecise(1.23))
console.log(isPrecise(1.2))
console.log(isPrecise(1))
console.log(isPrecise("1.23")) //works if its a string too

You can also use a regex:

function isPrecise(num){
  return /\d+\.\d{2}/gm.test(String(num));
}

console.log(isPrecise(1.23))
console.log(isPrecise(1.2))
console.log(isPrecise("1.23")) //works if its a string too

but toFixed turns it into a string, and if you use parseInt it gets rid of the trailing zero

You can have a number or you can have a representation of a number, but there is no third alternative.

If you want a representation of a number, what's wrong with a string? If you want a number, then there is no "trailing zero".

The number represented by "2.0" doesn't have a trailing zero because only representations can have trailing zeros. The number two can be represented with or without a trailing zero and it's the same number.

So: Do you want a number or a representation?

I think you must want a representation, because really only representation can have some specific number of decimal places. In that case, why isn't toString the right solution?

Number.prototype.countDecimals = function () {
    if(Math.floor(this.valueOf()) === this.valueOf()) return 0;
    return this.toString().split(".")[1].length || 0; 
}

When bound to the prototype, this allows you to get the decimal count (countDecimals();) directly from a number variable.

E.G.

var x = 23.453453453;
x.countDecimals(); // 9

It works by converting the number to a string, splitting at the . and returning the last part of the array, or 0 if the last part of the array is undefined (which will occur if there was no decimal point).

If you do not want to bind this to the prototype, you can just use this:

var countDecimals = function (value) {
    if(Math.floor(value) === value) return 0;
    return value.toString().split(".")[1].length || 0; 
}

EDIT by Black:

I have fixed the method, to also make it work with smaller numbers like 0.000000001

Number.prototype.countDecimals = function () {

    if (Math.floor(this.valueOf()) === this.valueOf()) return 0;

    var str = this.toString();
    if (str.indexOf(".") !== -1 && str.indexOf("-") !== -1) {
        return str.split("-")[1] || 0;
    } else if (str.indexOf(".") !== -1) {
        return str.split(".")[1].length || 0;
    }
    return str.split("-")[1] || 0;
}


var x = 23.453453453;
console.log(x.countDecimals()); // 9

var x = 0.0000000001;
console.log(x.countDecimals()); // 10

var x = 0.000000000000270;
console.log(x.countDecimals()); // 13

var x = 101;  // Integer number
console.log(x.countDecimals()); // 0

Using modulus will work:

num % 1 != 0
// 23 % 1 = 0
// 23.5 % 1 = 0.5

Note that this is based on the numerical value of the number, regardless of format. It treats numerical strings containing whole numbers with a fixed decimal point the same as integers:

'10.0' % 1; // returns 0
10 % 1; // returns 0
'10.5' % 1; // returns 0.5
10.5 % 1; // returns 0.5

I think my code is perfect ...

/**
 * @param {string} s
 * @return {boolean}
 */
var isNumber = function(s) {
    return s.trim()!=="" && !isNaN(Number(s));
};