To be fair, its stupidly easy to fake the sender address of an email- and all KLM are doing there are making it easier to pick a valid looking source email address...

The only sensible way is checking where the links go, and making a decision about whether or not you trust that destination.

For me, the fact the sender knew about a recent flight would nudge me into possibly trusting it, but certainly not the sender's email address being on list on KLM's website.