Assuming that you are using presta, you'll need a gauge that goes on a presta valve. There are any number of them around. Their accuracy isn't that much better than your pump's but you'll have a second data point. If you have a schrader valve, you could use a car tire gauge or get one specifically for bicycles. There are several models of pressure gauge that will do both presta and schrader. I've used a Zefal in the past. They are inexpensive, analogue and have a feature that stops the needle at the pressure so that you can read it off the bike.