The best way to check air pressure on a Presta valve is to hook up a floor pump with a built-in gauge and top it off at whatever pressure you want to ride (probably just a few strokes). Just takes a second, and your tire will always have lost air pressure just by sitting around, anyway.

Pretty soon, you will get a "feel" for how often you should do this. I used to do it daily, but now about every 3 days or so.