How can I see which moderator accepted/declined my flags?
I doubt this is possible now, but I wanted to ask. If not possible, then what's the reason for keeping this secret?
In the end, how could we evaluate the activity of a moderator?
You can't see that information, and I doubt SE will ever change that.
It is generally much better to focus on specific actions than focusing on people when complaining about the moderation on the site. When such discussions get personal, they tend to escalate and become far less constructive. Discussing the actual issue without assigning blame first can avoid that in most cases.
At first it doesn't matter which moderator exactly declined your flag, what matters is why they declined it and whether that action was correct or not.
A single declined flag is nothing to worry about, but if you get multiple flags of the same kind declined, either you're flagging wrong or the moderator is wrongly declining them.
So if you think that your flags are declined for the wrong reason, make a meta post about it and explain why you think they are valid flags. If you're right, the moderator will likely adapt their behaviour and avoid making the same mistake in the future, problem solved. If you're wrong, you'll know why you shouldn't flag that kind of post and will stop that, problem solved as well.
Now, there could be a case where a moderator makes consistently bad decisions on flags and is not willing to change. If users complain about specific flags on meta, that kind of behaviour won't go unnoticed by the other moderators and even SE at some point. The moderators and SE also have the full picture available, and they have the power to do something about the issue if the moderator can't be reasoned with.