Hi Mark,
Understood, the most basic way to measure that directly would be as Bob mentioned, a DMM with high resolution (some meters have a function to capture the max meaurement - giving you a worst case, most drawn number).
I think that UNI-T clamp meter ought to have it(I got one on a whim and it has been good so far).
The most advanced is with a power measurement device like the Otii Arc: Qoitech Otii Arc Pro | Buy in Australia | CE09636 | Core Electronics
It captures the fluctuations as the processor is running. (other options are available, but I’ve used the Otii with amazing success).
Both of these options give a calibrated, repeatable way to estimate battery life.
Boards like this will allow you to read the battery remaining: Adafruit LC709203F LiPoly / LiIon Fuel Gauge and Battery Monitor - STEMMA JST PH & QT / Qwiic | Buy in Australia | ADA4712 | Core Electronics
I recall one also offered current measurement but I wasn’t able to track it down
There have been some interesting topics recently on this. Donald (Connect solar panel to ESP32's 5V IN pin - #4 by Liam) has a good summary in recent times (it is also worth taking a read through some of the regulars posts over the years).
If you do go the route of an external (Pico attached device) imo it would be worth using an I2C device or similar that offers readings like average, max current readings.
