Some circuit boards have the polarity marked on them in either the silkscreen or the copper. I don't know if the Atlas board is marked. Some LED packages has a small flat area on the side at the + lead. Sometimes, you can find documentation on the board that tells connections and polarities of components.
In the electronic devices department, an ohm meter may allow you to determine the polarity but it may not. The high forward drop (3.3 V) of most white LEDs is too high for many analog meters. Meters with 9 V batteries may damage reverse connected LED. A simple testor can be created with two 4.7 v to 5.6 V zener diodes connected in parallel but in opposite directions, a resistor connected in series with the zener diode combination, and either a 9 V battery or a 12 v dc supply (a good power pack for the track) to apply power across the entire assembly. The cost of the circuit less battery or power supply may be $3 or less from a Radio Shack. The unknown led is marked to identify which lead is connected where to the board. The unknown LED is removed and connected across the zener diodes and power is applied. If the LED lights, the positive lead of the led is towards the positive of the power source. If the LED does not lights, the negative lead of the led is towards the positive of the power source.
The theory of operation is simple. The paralleled opposing zener diodes clamp the voltage to avoid damaging the LED with excessive reverse voltage while allowing the LED to light at 3.3 v if it is properly connected. The resistor to limit the current through the LED to a safe value. For a 20 mA LED current, a 3.3 V led voltage, and a 12 volt power supply, the resistor would be: (12-3.3)/(0.020) = 435 ohms I'd recommend a 470 ohm resistor since it it the nearest standard value in a
10% tolerance. The power dissipation on the resistor would be (12-3.3)*(12-3.3)/470 = 0.16 watts. I'd use a 1/2 watt resistor.If you need more information or want a simple circuit diagram, let me know and I'll email a .PDF of the circuit and sample calculations to you.