In the RSA DES challenges, how did the contestants know they had found the right key considering they weren't given any plaintext?
One can still access the challenge rules from the archive.org


  Each contest is based on a specified cipher. A brief piece of printable ASCII text (containing byte values in hexadecimal notation from 0x20 to 0x7e) will be appended to the fixed 24-character string "The unknown message is:". The result will be padded and then encrypted with the associated cipher under a randomly-generated key.


The message is also padded by PKCS#5 padding. Thus, the attacker has three options to test their results;


Check the beginning of the message; it must start with "The unknown message is:"
A valid PKCS#5 padding at he the end.
And, one general rule when only ciphertexts are given; one can check the result is valid (English) language. For this, one block may not be enough, since there are many valid words in the 64-bit block. One block will result in many keys. To narrow the keys, additional ciphertexts are required. This approach can be work even 1. and 2. cases are not known. For this specific challenge the range of the characters given between 0x20 and 0x7e, i.e. between  and ~ 




distributed.net  found this message in DES Challenge II-1;


The secret message is: Many hands make light work.




Note : It is interesting that the Wikipedia says; The secret message is: where the source is lists.distributed.net, however, the RSA archives says The unknown message is:.