-Answer Amen immediately after hearing a blessing being concluded by another person, BUT Do not say Amen after your own blessing.
Q- My question is...what is done if you eat alone or eating with someone who does not say blessings, or know your customs?
Answer You only amen blessings said correctly by other people. By saying amen you are voicing agreement. As such, if you are the only one saying the blessing or the only observant one involved with the activity no 'amen' is appropriate (no one is agreeing with the speaker). Hope this helps. Shalom!