Since Apple hid the charging current info (that previously was available before iOS 10) there isn't a way to see the charging current that's going into the phone (gee, thanks Apple).
But you can do a test to find out what charger puts out the most juice - time it.
Bigger chargers will charge an iPhone faster than the dinky 5 watt charger that came in the box.
Discharge the phone to 50% (nice round number) then turn on airplane mode so the phone doesn't get distracted, set auto lock to never (so the display stays on continuously, then charge it up to 70% while timing it. Don't use the phone during the test except to look at the lock screen. Do that test with both the 5 watt charger (for a baseline) and your other chargers.
Don't time the test above 80% because the charging circuits inside the phone start tapering the current. A tiny charger and a big charger will charge the phone at about the same speed once the charge level hits about 80%.