Perryn Fowler has a few words of wisdom to share on what can and cannot be tested. I especially like his closing remark.

His remarks are short, so I have reproduced them here in full.

some things you just can’t test

Rubbish. Think harder.

Of course some things are very difficult to test, and in some cases the effort is not worth it when compared to the amount of risk it would mitigate - but that is a business decision rather than a purely technical one.

You can make this decision a lot easier by not putting important behaviour in things that are hard to test