General Help: Testing Payment Methods
Help for a specific addon is to be found on the product page - testing help is generally in the User Guide.
For testing card payments, each payment provider has a set of card numbers to use. In many cases there are different numbers to use for different results, others have alternative methods e.g. on buyer name. If you don't find a link to the test card numbers in the User Guide, contact me.
Testing Apple Pay
The Apple Pay button only appears on devices that support Apple Pay. This means you need an Apple device on which to do your testing. If you don't already have one and can't borrow one, you will need to buy one. You can get a second hand one without spending too much but make sure it's not too ancient … check the model against Apple's list at https://support.apple.com/en-gb/102896
You should not enable Apple Pay as a payment method without testing it. If the domain association file is not properly configured, the button will appear but if a customer uses it they will not be able to pay. Most payment providers do not have an API call that the addon can use to check the domain association is properly set up.
You are going to want to test Apple Pay with a small value order using the payment module in live mode not sandbox mode. This is because although Apple has a testing environment, you need an Apple Developer account to use it and they cost $79 per year.
Browser Console, Inspect, Debugging on Mobile Devices
Short answer - you can't. To do it natively you need to connect to the right sort of computer; for iOS you need a Mac and for Android a computer running Chrome.
I found the Android / Chrome thing a bit of a pig to set up and I was not about to go out a buy a Mac (even a used one) just for debugging so can't comment on how easy that is. Instead I found a great app that allows you 15 minutes debugging free per day on each platform. If you're in a hurry or do a lot of development the unrestricted Pro subscription is $79 per year. You connect your mobile device to your Windows, Mac or Linux computer and get the Chrome Dev Tools interface to debug.
