The current issue of IR Watch features this question. In a nutshell:

1. View the source code of an email and identify the time stamps from any two mail server records.  Time stamps are often given in the following format:

hr:min:sec +/- GMT

where hr is hour, min are minutes, and sec are seconds. +/- GMT is the Greenwich Mean Time difference which can be positive (+), negative (-), or neutral (0).

2. To compute the time difference between the two records, do as follows:

Step 1: Convert time stamps to GMT times.

Step 2: Convert the new times into seconds.

Step 3. Take differences.

You should be able to convert this difference back to  a hr:min:sec format

This can  give you an idea of the estimated email delivery time between the two records; that is, if you trust email headers information.