Garraghan divides criticism into six inquiries (A Guide to Historical Method, 168):

1 – When was the source, written or unwritten, produced (date)?

2 – Where was it produced (localization)?

3 – By whom was it produced (authorship)?

4 – From what pre-existing material was it produced (analysis)?

5 – In what original form was it produced (integrity)?

6 – What is the evidential value of its contents (credibility)?

The first four are known as higher criticism; the fifth, lower criticism; and, together, external criticism.

The sixth and final inquiry about a source is called internal criticism.

For more information about the application of these principles:

http://vccslitonline.cc.va.us/MRCTE/intextcr.htm

http://www.okstate.edu/ag/agedcm4h/academic/aged5980a/5980/newpage19.htm

http://72.14.235.104/search?q=cache:pvmdOOc1cCEJ:www.ups.edu/documents/HIST200IntCrit.doc+external+internal+criticism&hl=en&ct=clnk&cd=28

Hope you find these helpful! Good luck!