As a long as your app is not secure itself there is no better chance than going through the logs and see if there were requests with a referrer from an unknown site/site that makes not much sense. But this only works, when all the request headers are logged.
Once your app is secure, somewhere will be code like this
if is_csrf_attack
log_csrf_attack
else
go_on
end
Edit: With "making not much sense" I meant that you should look for URLs in referrer headers that should not send requests to your site -> it makes not much sense for these sites to call your API/URLs.