we did it two ways on the two cars in our shop. on mine i was replumbing the cell so i put a tee right at the pump outlet and pluged the tee. very safe but you need a jack to get at it. the second car has a pressure regulator in the engine compartment and we just unscrewed one of the fittings and added a tee and plug. with either solution you need to have a barb fiting and piece of hose in your tool box to draw a sample. both cars passed tech last weekend.
dick