How to Playtest

[PaintByBlood]

I have a lot of ideas, but not quite so much experience.
Many things I want to make, but an understanding of the impact that different features have that still has a lot of room to grow...

Are there any sources on how to thoroughly playtest things, especially classes? I've searched, and finding anything seems to be very difficult - most possible things being overshadowed by links about playtesting upcoming D&D stuff.
Or, also helpful, any advice from more veteran homebrewers?

[Azoth]

A basic way is to design several encounters of varying types (environment hazard, mook mowing, boss fight, social interaction, ect ect) and run the homebrew through it. This can give you an idea of the versatility and power of a class. When I do this, I try to pick at least a few encounters where the designed schtick of the class won't be effective so as to see what the class is capable of when its strongest tool is removed from its arsenal. Then compare the results against those you gain by running classes of various tiers optimized to the same level through the same encounters to see the general effectiveness compared to a given tier.

For races a good way is to compare it to races of a similar LA. Obviously excluding those that borderline or are considered under/over priced. So do not compare your LA1 race against something like the half minotaur, lolth touched, or similar.

Aside the above methods which only give data in a proverbial vacuum. You can simply introduce it as an option for a "tester" campaign to see how it is played in a real campaign and what balancing point settles at.

[Kane0]

You might find this thread handy, you can sign up and test your brew on the challenges they present there.

[PaintByBlood]

Thanks for the tips, and I'll be sure to keep an eye on that thread.
I think the main thing I'll need to do, at least for the class I'm thinking of, is get some people together to do a test campaign, and see how it fares there. It wasn't designed to be versatile, more to fit a single archetype that one might want to play, and I think the main things I'd want to discover are if it is tier 4/5 (probably isn't any other), and if it does what it does too well and would make other classes feel useless when it was involved...
A campaign would probably have to be geared slightly toward it to make it a feasible selection for most people, but I don't really see that as a problem or anything, just a fact of the archetype.

[Yitzi]

When I do this, I try to pick at least a few encounters where the designed schtick of the class won't be effective so as to see what the class is capable of when its strongest tool is removed from its arsenal.

Of course, sometimes (particularly if you're aiming for tier 5 or a specialized* tier 3 or 4), you don't want it to be capable of much in encounters other than the one it's designed for.

*Note that a specialized tier 3 or 4 should be roughly as good at its specialty as a tier 1 or 2