Clues that you're dealing with base64 encoding--
so let's look at the payload where
the red arrow is pointed to.
We can see it starts with Rk9SN and that it
ends in these equal signs.
The equal signs at the end, these
are padding for that 6-bit representation.
If you see that, that's a clue that you're
dealing with base64.
Another clue is if you scroll down
that column or the messages, you would
see that it starts with the same characters.
That is not common with encryption.
Now, could you have something that
has equal signs at the end that is true encryption?
Absolutely.
Just make sure you look for those equal signs.
That's a really good clue that something is base64-encoded.