How to only accept numbers in the input for cin?

Question

I'm making a very simple calculator (only two numbers that are either added, subtracted, multiplied, or divided) c++ program and I'm nearly done except one line of code. I want to make my program display an error message if anything except a number is entered...but I have zero clue how to. Also pleas keep in mind that I'm a beginner at c++ and I don't want to use anything too crazy. I tried typing in multiple different ways of asking this question, but found nothing. If there's a question already similar to this please let me know and I'll take this down. The line in question is commented out, it's the 6th line into main(). Thanks in advance for any help :))) Here is the code:

    #include <iostream>
    #include <string>
    #include <cmath>
    #include <iomanip>
    using namespace std;

    string calcSign;
    double result;
double cAdd(double num1, double num2);
double cSubtract(double num1, double num2);
double cMultiply(double num1, double num2);
double cDivide(double num1, double num2);


int main()
{
   double firstNumber, secondNumber;
    cout << "Hello! Please enter first number: " << endl;
    cin >> firstNumber;

    /*
    if (firstNumber != ){
        cout << "**ERROR...Please enter a valid number";
    }
     */

    cout << "+, -, *, or / ?";
    cin >> calcSign;

    cout << "What is the second number?: ";
    cin >> secondNumber;

    // If/else logic to determine what math operation to use
    if (calcSign == "+") {
        result = cAdd(firstNumber, secondNumber);
        cout << setprecision(2) << fixed << endl;
        cout << firstNumber << " " << calcSign << " " << secondNumber << " = " << result << endl;
    }
    else if (calcSign == "-") {
        result = cSubtract (firstNumber, secondNumber);
        cout << setprecision(2) << fixed << endl;
        cout << firstNumber << " " << calcSign << " " << secondNumber << " = " << result << endl;
    }
    else if (calcSign == "*") {
        result = cMultiply(firstNumber, secondNumber);
        cout << setprecision(2) << fixed << endl;
        cout << firstNumber << " " << calcSign << " " << secondNumber << " = " << result << endl;
    }
    else if (calcSign == "/") {
        result = cDivide(firstNumber, secondNumber);
        cout << setprecision(2) << fixed << endl;
        cout << firstNumber << " " << calcSign << " " << secondNumber << " = " << result << endl;
    }
    else if (calcSign != "+" || calcSign != "-" || calcSign != "*" || calcSign != "/") {
        cout << " ** Error - Please Enter a Valid Sign" << endl;
    }
    return 0;
}

// Two numbers multiplication constructor function
double cAdd(double num1, double num2) {
    double result1 = (num1 + num2);
    return result1;
}

double cSubtract(double num1, double num2) {
    double result1 = (num1 - num2);
    return result1;
}

double cMultiply(double num1, double num2) {
    double result1 = (num1 * num2);
    return result1;
}

double cDivide(double num1, double num2) {
    double result1 = (num1 / num2);
    return result1;
}