IN a lobby you see 4 green dots, press A/X on someone and wait a few seconds to see if the meter drops to 3,2 or 1 aka ping check, in ranked you get throw in with any random willy and same with regular matches.
AKA ALWAYS go into a room and find someone with a good ping, but even then you can still get enough lag to drop combos and button inputs, yes, you can drop moves if you do them as quick as you do offline like there is no button input buffer.... lol.
Online in MK is a joke, but it can still be fun against someone with a good ping, just not perfect.