A level chemistry - orbitals

I’m revising atomic orbitals and I’ve come across an answer to a question in the WJEC AS textbook that contradicts the information that is previously said. The textbook says that the 4s subshell fills before the 3d subshell. The question in question states ‘write the electronic configuration in terms of sub-shells for a chromium atom’. The ‘answer’ is 1s2 2s2 2p6 3s2 3p6 3d10 4s1, contradicting the information previously given as the 3D orbital fills before the 4s. In the information in the textbook wrong or is the answer to the question, or am I being stupid and not seeing something otherwise obvious?

It's not obvious. You can write the electronic configuration in two ways - in order of filling (like most students prefer to) or in order of quantum numbers (as the answer is suggesting). Some exam boards accept either, not sure about WJEC.

Ok, how would I know if I got this kind of question in an exam whether to answer in terms of filling or in terms of quantum numbers?

You need to look at markschemes for that - they will use one or the other, and some (most?) will accept the alternative. This is exam baord specific stuff