As for the crosscheck against the literature this is done in the tutorials.
https://agnpy.readthedocs.io/en/latest/tutorials/external_compton.html
@jsitarek though pointed out that he cannot reproduce results in some old MAGIC publications
https://github.com/cosimoNigro/agnpy/blob/master/tests/crosschecks/ec_js.ipynb
we have to check this before marking the tick box.

crosscheck against back-of-the-envelope calculation where started by @jsitarek in
https://github.com/cosimoNigro/agnpy/blob/master/tests/basic/simple_ec.ipynb
and commented in Issues #12 and #13.

I think the introduction of the test suite addresses the pending checks in this issue.
We have now the following automatic crosschecks for the external Compton radiation:
- crosscheck for EC on Disk against the literature (Finke 2016)

- crosscheck for EC on Broad Line Region against the literature (Finke 2016)

- crosscheck for EC on Dust Torus against the literature (Finke 2016)

As for the crosschecks against back-of-the-envelope calculation I have implemented two test functions that check, in the limit of large distances, if the EC on the BLR (or DT) tends to the one of a point-like source approximating them.

The crosschecks against another code are a bit difficult since Jetset uses different geometries for the targets.
Nonetheless I am confident the External Compton calculations in agnpy are reliable.