Is there a reliable way to reject incoming mails with a spoofed e-mail address?
Over the years, there are some techniques implemented to identify spoofed email address, for example: SPF, Sender ID, DKIM, and DMARC. Take a look on these two excellent answers about email spoofing on security.stackexchange, here and here.
What kind of checks does postfix run normally on incoming mails?
To find out what default config shipped with postfix, please run
postconf -d. And then take a look on smtpd_*_restriction. As default, postfix just run two checks,
Parameter permit_mynetworks is permit email from your network. Parameter reject_unauth_destination is basic setting to make sure that your server doesn't became open relay, visit this page for brief explanation.
does postfix check the reverse dns by default?
Yes, but postfix only gave warning when reverse dns check fail. Here the sample warning from maillog.
Aug 22 10:37:17 mx postfix/smtpd: warning: hostname st.example.com does not resolve to address 192.168.231.235
does postfix have any other checks built-in and activated by default?
Check this page, and look for entry smtpd_*_restriction (e.g smtpd_client_restriction, smtpd_helo_restriction and so on). There are many check on postfix but as I said above, by default only two checks enabled.
what kind of filters / milters are useful to prevent accepting spoofed mails?
There are some milter/third apps to provide assists postfix battle spoofed email by applying four techniques I said above. For example tumgreyspf, opendkim, opendmarc and many others.