- Forums and Question/Answer sites might be your widest net option.
- A new app called GameFor may be what you are looking for as well.
- Local gaming stores can offer some great possibilities.
- Keep your pitch short and sweet to avoid losing your audience.
- Make sure you don't hover when they are testing...
- Make sure you take any criticism as intended...
- Testing online with Roll20.net/Skype could be key for you.
So there are of course a great many ways to attract potential players to play test a game you are developing. It all depends on the type of play group you are searching for and how you want to test it. Online, In person, or Both.
While I have not actually used any of these methods to playtest a game it is not a stretch to see how they can be beneficial. All you are doing is finding a group to play a game (which I have used these methods for), but not participating with them...
To find individuals to play you have no limit to where you can go to find them. You can post something online looking for interest on forums, Yahoo Answers, Reddit, Tumblr, even this stack's chat room. Better yet, you can post in all of those places and hopefully generate even more interest.
The key here is to just find someplace that you can ask and individuals can respond. I have found people to play online with through Yahoo Answers before and website forums. Any place that you can post your request and receive multiple answers will work... even craigslist if you wanted to go that route...
There is also a new app that has recently been released that may be a big help for your endeavour. The GameFor app (both iPhone and Android) is geared exactly for what you are searching for. Finding gamers to game with all around the world. You can find people in your local area, as well as local stores and events, and meet up to play. The only downside currently is that it is still only a few months or so old, but the more gamers that get on it the bigger it will grow!
As I mentioned this app is new, and as such I have not used it yet, but there are not just individuals that appear "near" you. There are also entire groups and game stores.
Another option (I think the best, first option) would be to go to your local gaming store. This has a lot of great benefits. First, you can meet gamers face to face and see their reactions while reading or playing your game as opposed to just reading reviews in emails. Also, you might be able to get the store owner or employees to help spread the word if they think you are onto something!
I have also joined game groups through stores. Just going to a store you can enter, talk with the owner or players that are there, and quickly generate interest on the spot. The owner (to my experience) typically knows the type of gamers that come into their shop and might be able to get you in touch with exactly the type of player you are looking for.
Whether you post online or give your pitch in person I would try to keep it short and sweet. Explain the basic storyline, basic play style, and any unique game mechanics you have. Beyond that, I would advise that those interested further can contact you for more details/information (given that those interested will have to contact you eventually to play regardless).
If you get a slew of responses this could be even better because you can set up multiple play test groups to run at the same time. If you do, of course, try to check in on all of them and ensure they are running as intended, but also try to withdraw yourself and ensure that someone picking up the "book" the first time can figure it out without a lot of direction.
If you do want to limit interaction to cyberspace (or simply have no other viable choice) Roll20 is a great resource for playing RPGs online. With Roll20 and Skype I have played numerous games online with no additional cost to my wallet or meeting another human.
Whether you have groups play test online or in person you can still lurk in the background and take notes for yourself on what you observe. Maybe you will more easily notice something that a player or DM would miss: "That NPC should not have done that, there is a situation where my game mechanic could fail, etc."
Most important however I think would be criticism you receive. If someone takes the time to test your game, make sure you look at ANY criticism you receive objectively. Since it is your baby you may be inclined to defend it and provide excuses, but looking at your project objectively is the only way to ensure it improves!