PEEK is a general-purpose evaluation monitor that records and outputs any input values that are passed to it. This is useful for monitoring the function evaluations performed by general numerical routines such as QUAD or FMINSEARCH.

PEEK works in much the same way as SOW/REAP in Mathematica.

Basic example:

f = @(x)sin(x);
fpeek = @(x)peek(f(x),x);
quad(fpeek,0,pi);
[fq,xq] = peek()

See the help file for further examples.