Unfortunately, they're smarter than above. But it's usually quite easy to spot. There are technical terms where translation tools fail completely.
Some examples (for french):
Mining pools: translation tools often translate 'pool' into 'piscine' which means swimming pool. And we end up with ridiculous translations saying: Mining swimming pool.
Whitepaper : the literal translation is 'Papier Blanc'. But in good French, we say "Livre Blanc" (White Book). So when I see 'Papier blanc', it makes me very suspicious.
If I have any doubts, I compare with the original ann and look at how I would have translated it. Or sometimes I use the translation tools and if the result is the same (and with incomprehensible sentences), I'm pretty sure it's an automatic translation.
And I also rely a lot on the number of reports I receive about an ann. If several (trusted) members tell me that it is an automatic translation, my investigations are shorter and the sanction comes quickly.
Otherwise there is the possibility to open a topic in Reputation and ask people who understand the language you don't know. And you link this topic in the report and mod will act according to the answers of this topic.