How can i write code for Peck Detection for DSP by using derivative??

How can I write the code to find out the peck detection for DSP by using the derivative function? I am a very beginner. So that I have faced some problems. I have tried to write but now how can I find out the plot for derivative function i.e. 1st,2nd,3rd

clc;

clear;

t=0:1/200:2;

signal=sin(5*pi*t)+cos(13*pi*t)+sin(16*pi*t);

plot(t,signal)

hold on;

for i=2:length(t)-1

if signal(i)>signal(i-1)

if signal(i)>signal(i+1)

plot(t(i),signal(i),'o')

end

end

end

diff(signal)

diff(signal, 2)