Is there a way to check if a var is using setInterval()?

There is no direct way to do what you are looking for. Instead, you could set timer to false every time you call clearInterval:

// Start timer
var timer = setInterval(fncName, 1000);

// End timer
clearInterval(timer);
timer = false;

Now, timer will either be false or have a value at a given time, so you can simply check with

if (timer)
    ...

If you want to encapsulate this in a class:

function Interval(fn, time) {
    var timer = false;
    this.start = function () {
        if (!this.isRunning())
            timer = setInterval(fn, time);
    };
    this.stop = function () {
        clearInterval(timer);
        timer = false;
    };
    this.isRunning = function () {
        return timer !== false;
    };
}

var i = new Interval(fncName, 1000);
i.start();

if (i.isRunning())
    // ...

i.stop();