A good quality fitness tracker with a built-in heart rate monitor like a Garmin Forerunner will give you a reasonably accurate estimate of calories out. Your can also get an occasional DXA scan (low dosage x-ray) to measure body composition; the report will include an estimate of your baseline resting calorie consumption per day. Remember that as you lose weight your daily calorie consumption will also drop because you have less fat to carry around and keep alive.