Easiest way to tell is health regen. You regen a small amount of hp/tick depending on level (see:
https://wiki.project1999.com/Charact...P_Regeneration). If your health goes up more than that, that's not the server tick. When the real server tick happens, your hp will get knocked down to its true value (+ whatever hp from where it was previously, per the previous link). Then you can stand, do your casts, whatever, and sit down before 6 seconds elapse.