Being able to recognize OpenPGP messages is especially an issue as the recipient is included in the OpenPGP message's meta data by default (it can be hidden though, using the "hidden recipient" feature; which trades reduced meta data for more effort on the recipient's side, as he has to try all his secret keys instead being able to select the proper one from the beginning).
As far as I know, its not possible to detect the content of a mail, even just only if it is OpenPGP encrypted when the ways sender - mail server - mail server - receiver are TLS encrypted. Am I wrong?
You're right, as long as there is no flaw in TLS and proper algorithms/parameters are chosen, contents transmitted through an encrypted channel cannot be recognized.
Statistical methods on the message size might be applied to guess the contents might be OpenPGP messages -- but then again, OpenPGP messages are generally hardly distinguishable from "normal" messages with respect to their size.
Of course it can be detected by the mail servers and if one of the ways is not TLS encrypted.
You are also right about the mail servers being able to analyze whether a message is OpenPGP-encrypted, as they terminate the transport encryption through TLS. If you trust all servers in between (and there are often more than two), also be aware that you cannot make sure all mail servers on your message's route always communicate encrypted -- there is a rather larger portion of mail servers not encrypting connections between SMTP servers at all! This is especially valid for network-internal communication, which is rarely encrypted at all; but also an issue for cross-server-connections through the internet. Finally, even if servers have certificates, lots of them do not have a proper certificate, or do not bother to verify the other side at all.