What's the difference between voiced and "unaspirated" consonants and how can I check if I got it correct?
Voice-Onset Time

As you probably already know, the distinction between voiced vs unvoiced-unaspirated vs unvoiced-aspirated is the relative timing of articulation and voicing (called voice-onset time or VOT):


Voiced stops have a negative VOT (i.e., the voicing begins before articulation)
Unvoiced-unaspirated (sometimes called "lenis") stops have approximately zero VOT (i.e., voicing and articulation co-occur).
Unvoiced-aspirated stops have positive VOT (i.e., voicing begins after a delay relative to articulation).


In IPA, we use discrete symbols to indicate these distinctions (e.g., b/p/pʰ). Lumping VOT measurements into "negative", "zero", and "positive" is just a convenient simplification, though. In fact, there is a range of possible values that it can take on, and languages do, in fact, differ from one another in terms of how they realize "voicing" or "aspiration". However, any given language only distinguishes at most 3 stops on the basis of VOT alone**, so IPA (e.g., b/p/pʰ) is sufficient to describe things.

Measuring VOT

This is all great, but you say that you're learning Japanese (which has very negative VOT on its voiced stops), and as a speaker of Mandarin (zero VOT) and English (slightly negative VOT), you're not sure whether you're producing voiced or lenis stops. You can in fact measure VOT on an audio recording of yourself. Here's a tutorial on using praat to measure VOT on audio recordings.

If that sounds too complicated, you put your hand on your throat and feel when you produce voicing. To practice making voiced consonants having very negative VOT, try to keep your lips closed while starting vibrate your vocal chords. It's hard to do this very long, as your mouth starts to fill up with air, but it can help you see how it feels to begin voicing earlier than releasing a stop.

* It's probably even more complicated than that, but I'm going to assume that VoT is good enough to explain everything.

** Hindi "voiced aspirated stops" are breathy voiced, to respond to the obvious rebuttal.