How do you find out what the proper props would be for foamies?

You need to know what kind of motor, esc, and battery you have. Then you can determine what kind of props can safely be used with that system to get the thrust needed to fly the plane. How much thrust is needed for a particular plane? That depends on the type of plane, its weight, and the type of flying to be done with it.

Browse RCU's entire list of forums and pick out the style of flying that resembles what you are interested in.
APC has practically an infinite selection of props. They have way more props than what you see listed in the usual retail catalogs. Visit their website and call them directly to place an order once you know what you want.

Short answer: you read the manual that came with said foamy and see what the recommendation is. If the plane is RTF the manufacturer has done the work for you of picking the right prop. If it's something you are building yourself, the motor manufacturer will have a range of recommended props for you to choose from.

Longer answer....

The PROP not the motor, electronics, battery, etc... determines how much power will attempt to be drawn from your electronics.

The wrong prop will damage or destroy any setup, or cause your configuration to perform poorly.

With electric planes you start by taking the all-up expected weight of the plane, adding a factor, then working from that to determine the prop size, and subsquently motor, esc and battery to use.

As just pointed out, the manufacturers of RTF and many ARF planes have already done most of the work for you with their specifications, but of course what they concieve of is usually not what the purchaser may expect.

I thought you used a watt meter to test and make sure the motor and ESC were staying within the range of the specs? That is a question but I was told the props in the range should be tested with the meter to make sure you weren't over or under on the watt draw? I'm up on my prop selection for glow and gas but I'm still trying to learn the electric operation.

I always use a Watt Meter on every electric powered plane I have. Even though the factory may list specs that worked for them, I'll still like to check and make sure what I'm using really falls within the specs of the motor and ESC, and they are functioning as they're supposed to. Last thing I care to have happen is the have ESC let out that magic smoke it runs on in the middle of a flight. I also like to try different props (brands and sizes) to see what performs best for my style of flying, not what worked best for a test pilot at a factory. Using a Watt Meter also lets you determine roughly how long your flight time will be so you can set a timer accordingly and have less of a chance to damaging a battery.

Hogflyer

Thanks Hog, that is what I have been told, even shown how once when my flying partner bought one. Now I have forgotten how to use it but we have one. Prop testing on electrics has much narrower room for error. I spend most of a day testing props on my gas and glows but I don't do it much on my electrics. The thought of that smoke has me worried.

This is the "glow engine" approach which really doesn't apply to electrics. With a glow ( or gas ) engine, you could afford to switch props and see which prop produces the best result.

If you do the same with an electric motor, you could easily blow the electronics without even being able to see the draw from the prop.

Watt meters SHOULD be used to check that everything is within range, but only if you KNOW that you are UNDER to begin with.
Even then you are still risking your setup if you've overpropped a motor, not accounted for the draw, etc..

All the watt meter does in this case is give you a number just before your ESC blows or your motor wires melt. Remember that it can only take a second or two to happen!

On the other hand you can purposely choose to underprop the motor/ESC/Battery combo and use a Watt meter to keep trying different props while moving SLOWLY up in size.

The proper way to do things however is to size up your electronics PROPERLY from the start, selecting the operating wattage range and power system choosing a prop/RPM that will produce the draw you want.

More often than not the most efficient and powerful setups tend to run fairly close to the maximum capacity of the electronics and making a mistake with a prop is fatal, Watt meter or not!

The iPas Addiction-X setup for instance is very powerful with a lowly 3S LiPo but if you do not use their special "T" prop and instead use anyone elses of the same pitch and diameter you'll blow the electronics.... They have shaved things just that closely for optimal power output.