What you might find useful is to get a USB digital multimeter. They are very inexpensive on Amazon and the one that I got seems to work very well. $19 at the time. I’ve checked it against my regular digital multimeters and the USB one is pretty accurate.
https://www.amazon.com/gp/product/B07X3HST7V/ *** (see added note at very end)
The USB device records and remember the readings, even when powered off. Since it’s probably writing to some electronic memory, it’s going to wear out eventually but it’s great while it lasts. The instructions aren’t so great and it’s a little kludgy to use (read the Amazon reviews) - plenty of other models on Amazon, too, to choose from.
It’s very useful just to tell if any current is flowing to your device. I have a 2015 device where the micro-USB contact is wearing out and if I don’t seat the charging cable properly, the device may not charge. Interfacing the USB meter in the current path makes it easy to tell in plugging in cables whether current is flowing. Similarly good for testing cables and AC adapters for USB to make sure they are delivering the voltage and amperage promised, etc.
Also, if you have a USB-powered battery pack that has no LED charge indicators and you know what the device’s capacity is and want to charge up the Li-ion battery to about 50% capacity for long-term storage (better for long-term battery lifespan than to store 100% charged), the digital USB multimeter is very helpful. I learned an important thing to keep in mind, though. Battery capacity of a Li-ion battery in terms of mAh is usually calculated in terms of delivering current at 3.7 volts (the average voltage of a single Li-ion cell during its discharge cycle). But when you are charging, you are delivering USB current at 5 Volts. What’s important in determining how much energy goes into a battery is not Volts or amps separately but the combination, Volts times amps = Watts, units of energy delivered per second.
So if your battery is rated at 2,000 mAh capacity discharging at 3.7 V average and you want to charge it to the equivalent of 50% capacity at 3.7 V (1,000 mAh), when charging at 5 V, you only need to put in 3.7/5.0 x 1,000 mAh or 740 mAh at 5 Volts input.
The check is 740 mAh x 5 Volts = 3,700 mWh (or 3.7 Wh) and so is 1,000 mAh x 3.7 Volts (3,700 mWh or 3.7 Wh). The same amount of energy was put in at 5 Volts that’s going to be taken out at 3.7 V average in discharging the Li-ion battery.
After the Texas power outage on Valentine’s Day, 2021, I bought a “25,000 mAh” battery pack. To see if I got my money’s worth, after I almost completely discharged the battery pack, I charged it back up at 5 volts and had to use the above relationships to see, Yep, the battery pack was as advertised. It took ~18,500 mAh at 5 Volts to charge the device up to 100% capacity as the 25,000 mAh rating was for ~3.7 Volts average discharge voltage.
*** (Added Note) One thing about the device that I got. At the time that I got it, it didn’t handle the highest level of fast charging (it can handle lower levels). And to handle the lower levels, you have to insert a little metal adapter into the main body of the device to accommodate the reading of increased power levels. So if your goal is to see how high-level fast charging is working for any device, e.g., iPad Pro, laptop, etc., be sure to check the specs on whatever USB digital multi-meter device that you might be interested in - mean this as a general admonition to anyone reading this post down the line - not advice to anyone in particular.