Online calculators such as Spark is pretty reasonable at exercise where the intensity can be objectively measured (eg. running and walking). For aerobic exercise more generally, it uses broad averages of intensity for that kind of exercise. For exercise where you are working out against the resistance of the machine rather than your own bodyweight (eg. elliptical, stationary bike) it is less accurate.
If you know the average heart rate during your workout, you could check it against www.shapesense.com/fitness-exercise/calcul
However, heart rate is not always a perfect predictor of calories burned - high blood pressure and some medications can lead to a faster heart rate than the exercise really warrants.
My general advice would be to go with the more conservative of the two estimates.
Another reality check is the fact that only very fit people can sustain figures of above 10-12 calories per minute for any length of time, and figures above 14 calories per minute are extremely suspect. (Although it is possible to maintain higher burn rates for shorter periods eg. 10-20 minutes)