I anyway can avoid possible botnets by using that page offline when creating papewallet. But to be sure that the nmemonic seed is real, I has to check by some onlinewallet.
At the end of that site there is some guide of using GPG key. I guess a can download the same address-generator code from github and verify the validity of the code by GPG. The guide anyway is slightly messy for me. Can someone explain it more simple way? If it is possible?
[1] Download the .ZIP file from -> https://github.com/moneromooo-monero/monero-wallet-generator. You can extract it on an USB, preferably a fresh one. It contains an HTML page that is used as generator and can be used offline.
[2] You can grab MoneroMooo's PGP key from -> https://raw.githubusercontent.com/monero-project/bitmonero/master/utils/gpg_keys/moneromooo.asc. Subsequently, you can use any PGP/GPG program to verify if the generator hasn't been tampered with. The program simply verifies that the generator has been PGP signed by MoneroMooo. You should see a message similar to:
I am not sure, but I think this step can also be performed offline.
[3] Once you have verified the seed and have all the compartments on a fresh USB, you can bring the fresh USB to an offline/airgapped pc to generate a paperwallet with it.
Does this answer all your questions?
EDIT: Included a screenshot how [2] should look like:
EDIT2: If you or anyone else needs additional help or a walk through regarding [2], feel free to PM me on IRC. I am at #monero on freenode. I'll compile a guide for it later.