Up to my knowledge, there is not any app to assess sedentary behaviour. However, it seems that ActivePAL is developing technology (SitFIT) to assess sedentary behaviour in an specific way.
i wokld recommend you take take a look a the following review paper by Schoeppe et al (2017) Article Apps to improve diet, physical activity and sedentary behavi...
They actually note 7 mobile apps, specifically focussed on sedentary behaviour. I hope this helps
The activPAL is perfect at differentiating between upright position and sitting/lying down. See:
Grant PM, Ryan CG, Tigbe WW, Granat MH. The validation of a novel activity monitor in the measurement of posture and motion during everyday activities. Br J Sports Med. 2006 Dec;40(12):992-7.
Ryan CG, Grant PM, Tigbe WW, Granat MH. The validity and reliability of a novel activity monitor as a measure of walking. Br J Sports Med. 2006 Sep;40(9):779-84.