r - How to find the sensitivity of a Monte carlo test?

I have written a code which outputs a p-value of a given test from n samples and m replicates. How would I find out how the sensitivity of the test changes as a function of m?

sensitivity<- function(m) 

I am unsure how sensitivity is derived from a sample and related to p-value? I have only ever looked at this in table forms (or matrices) - is there a method to calculate it? (without built in R functions)