The core idea is watching for signs of transformation when playing with those tadpoles. You'll start seeing effects right after getting the astral-touched one, which triggers around level 5-6 typically. Important to note that black veins appear on your face permanently once you hit act 3, and the wisdom checks get increasingly difficult up to DC 21 if you've been using them frequently

The safest approach is using the commune ability instead of consuming them directly since this lets you share powers with companions without extra transformation risks. When you're hitting those key story moments especially during night camps, keep an eye on your wisdom stat because failing those checks can force some pretty nasty outcomes

Maybe interesting - the recovery animations after using illithid powers give enemies a bigger window to counter, so time those abilities carefully. Recent patches made overusing tadpoles more dangerous with some forced transformations popping up, but you can still manage it by spacing out usage and keeping high wisdom saves ready. Just remember that every power you unlock makes those final chapter checks harder, so think carefully about which abilities you really need for your build