If you hear a bracha made in a time that has already past- you do not say amen. If you are watching a live video and you can be sure that there is no time delay- you can say amen. If you hear a bracha such as we should see mashiach come soon, you can respond amen because it is a bracha that doesn’t “expire”.
You can always think the word amen- it is the first letters of kel Melech Nee’man