I recently purchased a battery pack from an electronics market - it's
not brand name, and I doubt it meets the listed specifications.
I'm interested in testing whether or not it actually has a 15,000 mAh capacity or not. Since it only has USB ports, I'd have to find a way to connect them to an ampmeter, which I've never done before.
Given that you specifically say you do not have brand or specifications, so you cannot be asking a "tell me about this specific battery question
and you ask about testing
and you talk about test equipment and ask advice on its use.
Is this question on topic for this stackexchange site?
I would see this as very much a valid question for his site.
It seeks to learn about test methods.
It applies to a range of products.
Other users asking the same sort of generic question can learn from it.
However, it may be useful to note that a significant number of site users, for whatever reason, see one or some of the words words like 'battery or 'battery pack' or 'power pack' and seem to snap into a robotic shopping-question-consumer-bzzzt-must-close mindset, and as it only takess handful of these to get a question put on hold, such questions quite often get put on hold. More often than not terminally.
So, unfortunately, it pays to 'gild the lily' when asking such questions. eg major on pointing out that you are trying to design something, or test something, or develop a solution etc. Even if such may seem blindingly obvious to you and many others, as only a very few people are required to 'take such questions out of circulation' and make it much harder to get answers.
Reversing such decisions can be hard and, if somebody with adequate power decides to go with the flow, the question may even be deleted. The inherent merit of a question in such cases may well not 'save' it. So where possible it's best to steer clear of it happening in the first place.