Before you start playing you need to make sure, that you and your opponent use the same plugins and emulator-settings.
This checklist should help you, to compare your settings with the settings of your opponent, because if the settings are different
the game may work async.

You can print out this checklist (right click in this frame -> Print) and pin it on your monitor or whereever, because it's important for every online play.

Both players need to use the same BIOS-Version.
I recommend to use the version SCPH-1001.

Your Version : ______________________________

Both players need to have the same MemCard-content.
I recommend to delete the memcards while playing online
(or simply drag them out of the emulators MemCard directory).
If you still want to use memcards you will have to send your memcards to your friend.

Both players need to use the same Sound Plugin.
I recommend to use the emulators internal sound plugin.

Your Sound-Plugin : __________________________

Both players need to use the same sound settings.
I recommend to disable the sound completely, this is
the most "secure" option for a sync gameplay.

[  ]  Enable Sound
[  ]  Enable CDDA Sound
[  ]  Enable XA Sound
[  ]  Enable SPU IRQ hack

GPU- and CDR-settings shouldn't be important for sync gameplay, but you should still make sure that both players use the same subchannel setting in the CDR, and I also recommend to use GPU-plugins from the same developer (for example Pete's OpenGL- and Pete's D3D-Plugin works sync; but I am not sure if Pete's D3D- and Lewpy's Glide-Plugin will work sync.)

Options like 'Auto load ppf files enabled' should be disabled and the 'Country' setting should be set to 'Auto', if the game works async.

Play in window-mode, on this way you can still see the Kaillera-chat in the background and you can talk with your opponent.

If you have a feeling your opponent plays like the worst gamer on this world, talk with him about your points/lives/money or whatever, to make sure that the game is still sync. For example in Tekken, it could happen that you wins on your computer, and your opponent wins on his computer, that's a sign of async gameplay. ;-) ... in this case you should restart the game, and recheck your settings.