How can I estimate the amount of entropy (with JavaScript) that I'm seeding a CSPRNG with? I am trying to determine how much entropy is going into a specific CSPRNG in JavaScript, but I can't seem to find a pure-JS client-side implementation of an entropy estimator. 

So, I need to find a way to estimate entropy locally with pure JavaScript.

If it helps, the entropy is in an array which should be easy to manipulate.
If someone chooses to run your program with a seed that you can't predict but an adversary can, you won't be able to tell.  There is no way around this: nothing about the software can tell you what an adversary does or doesn't know; you can only use cryptography to ensure that if an adversary doesn't know a key and the message, then they can't decrypt the message.

In general, to estimate the entropy of a system, you need to have a model for how the system behaves.  This is how all entropy estimation tools work: they posit some parametric model for how the input data might have been generated; then use the data to fit parameters for that model; and finally spit out the entropy of the model with the fitted parameters.