There is an easy way to do this with no harm to your truck that is an offshoot of ShooterMcGavins unhealthy approach.

Take the truck to an empty parking and lock it up in 4WD.

Start out slowly turning in concentric circles. Baby steps a little at a time.

If you start to feel binding, you are most definitely in 4WD. If you make it to full lock with no binding, you have a problem.

The reason this works is that in 4WD your front wheels are turning at exactly the same speed and as you start to tighten up the circle the inside wheels will start to jump because the inside of the circle is shorter than the outside.

You will feel this immediately as it starts to happen. The truck will feel like it doesn't want to do what you are trying and if you push it to far it will start to buck.

If/When you start to feel the truck resisting what you are doing, stop. You have proven you are in 4WD. It's an unmistakeable feeling.