You can read their manuals

HMRC publish the manuals issued to investigating officers - this is in line with the Code of Practice on Access to Government Information.

The Enquiry Manual is here and a full list of other manuals is here.

Be warned: HMRC don’t disclose information unless they have to, you can see a cracking example here.

Also, these really are manuals and they can be a very dull (and technical) read. There are however some fabulous pieces of information and it’s well worth persevering. This - for example - is interesting, EM2001 - Working an Enquiry: Reviewing Earlier Years: Introduction states, “It is not effective use of your time to be pursuing cases involving small amounts into earlier years.” I bet that there are quite a few investigators who have forgotten that line!