The easiest way is to get a scale and press against it. That will give you the force in pounds of pressure. From there it's simple to find any other values.
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Kiteman gave a good pragmatic answer, but it will give you the total force from all your limbs and muscles involved in using the gym equipment.
If you really want a "scientific" answer, you need to isolate your measurement to just the one limb you're curious about. You can still use gym equipment, but, for example, use just one arm or one leg, and nothing else.
Try getting to the gym and see how much you can press etc.
So, esstentialy, however much you can benchpress? :D
Pretty much - if you can lift 10kg, that's a force of 98N