How does one calculate the truncation error of a given scheme?

For example, consider the heat equation:

A finite difference scheme for this is the DuFort-Frankel scheme, which gives:

.

How would I then calculate the truncation error of this scheme?

I know that, for the forward/backward/central difference methods of discretisation, you can get the truncation error simply from considering the 'extra' terns in the Taylor Expansions that you use to derive the schemes.

However, I don't know how the above scheme was derived, so how does one proceed?

I know from a textbook that the leading terms of the truncation error are, in fact: