[mattslight]

If you wanted to take a scientific approach you'd need to be able to do something like the following: (A) measure your actual work output and (B) some way of measuring your 'maximum expected output capacity'.

if A much less than B you are being lazy. if A is close to or equal to B then you are burnt out.

[The short answer of course is, if you are reading Hacker News you are being lazy]

As an aside; Pivotal Tracker - the feature / dev tracking software manages to define output using average number of work blocks completed over the past X weeks. How you measure your maximum expected output capacity might be more tricky to define.