That's really the only way....we all do it...and I've never heard anyone say they felt their beer was ruined because they opened the bucket to take a reading...
Remember there's a cushion of co2 protecting your beer. Just be conscious of how/where you set down your lid while drawing your sample...or better yet have someone help you. Sanitize both sets of hands, have a friend open the bucket...drop your theif or turkeybaster in to draw a sample, then have friend slam lid down... Should be able to do it in less than 30 seconds.