How confident am I to say the coin has been rigged Let's just say that I flipped a coin 100 times, got 80 heads and 20 tails, how confident am I to say that the coin has been rigged? Which statistical test should I use? thanks!

This is a binomial distribution. Check, e.g., this tutorial on how to formulate the null hypothesis ($p=0.5$) and what is required to reject it at a significance level $\alpha$.