Easiest ways you can check if you are experiencing this awful lag:

Access the Marketboard and see it takes a much while longer to load or fails to load repeatedly
Go to your retainers and see how long it takes for them to load their menus
Test on a training dummy and see if there is a big delay in your skills

If you have one of the three happen it's the delay caused by the NTT and it's an ISP issue. SE has been at fault at times, but the majority is sadly how your ISP handles everything.

I was tanking in Orbonne and I was worried the lag would cause me to fail a tank mechanic and wipe our group. Thankfully it all worked out but that stuff is absolutely obnoxious.