Do you know the sampling interval for the time signal? Suppose the sampling interval is pi/100.
t = 0:pi/100:40;
x = cos(pi/10*t)+0.5*randn(size(t));
plot(t,x)
Then you know where pi/2, 3pi/4 etc are.
t(50) is pi/2 and x(50) is the signal value at t=pi/2
plot(t,x); hold on
plot(t(50),x(50),'r^','markerfacecolor',[0 0 0],'markersize',10);
plot(t(75),x(75),'r^','markerfacecolor',[0 0 0],'markersize',10);
and so on.
