To measure things electrical and electronic, you need a variety of test and measurement equipment. For example, you can use a volt meter to measure voltage. To measure current, an ammeter is used. These tools are good for measuring static or slowly changing levels. For more complex measurements, you use more complex tools.
The Arduino as Test Equipment
What test equipment is available on your bench? Please don't feel like you must immediately go out and buy thousands of dollars of test gear (unless that's an option and this was the excuse justification you've been waiting for—if so, go nuts!). You may be surprised by what you can do with what's right in front of you.
For some baseline readings on Arduino performance, you ...