Measure the height of the liquid by placing a laser jig on the cap. (Volume may be a function of the height, so calculate a function first. V(h) ) You can use a Bluetooth enabled laser measurement tool like a Bosch GLM 50C. For extra bonus, use your wifi backscatter article to transmit the information.
Tool: https://www.amazon.com/Bosch-GLM-50-Bluetooth-Distance/dp/B015FCJTEA