With most respirators checking fit is pretty simple, cover the filter media (either with your hand or if too big a plastic bag will work) and try to breath. If you can get air with the filters (cartridges) covered it's leaking. Unless you've got a long full beard you should be able to adjust the mask to get a decent fit. If you have a full beard you need to wear a hood not a mask and you'll need a supplied air system.

Like Mike said, a quality mask (your life should be worth a good one) will have online guides for changing the filters/cartridges.

Also remember that for certain gasses, there are no cartridges that will filter them, like for isocyanates from many two part paints, you must use a supplied air system to safely use them or use them outside in the open air.

Bill H.