Check the size with a drill bit, then here is a quick and dirty way to check NG system pressure only, low pressure propane systems operate at pressures that would require a 18" deep container. take a bucket or other container 8 - 10" deep and fill with water, place in an outdoor well ventilated area and place end of hose at bottom, turn on NG and there should be no bubbles, raise end of hose towards water surface and measure distance of water above end of the hose when first bubble shows. This will be the gas pressure as measured in inches of water column, normal household pressure should be between 4" and 6" of water above end of hose in water bucket. All that is needed is a couple bubbles to confirm water depth then drop hose to bottom and shut off hose NG supply. If you try this with Low pressure LP gas it would start bubbling when there is 11 - 13" of water above hose end in water container.
This is not an activity to try indoors or in enclosed spaces as careless release of gas beyond a couple small bubbles may make you the star attraction on the local news channels when it ignites.