What are some methods for detecting a sinusoid in noise?
Depending on the frequency of sampling, a FFT (Fast Fourier Transform) will work. For example, if your sampling rate is only once every 20 ms, you'll only get a few samples of the sinusoid, but if you're sampling every 0.5 ms then you'd get a lot more samples. FFTs usually work best with a large number of samples to average over. In that case even if your signal is the sum of a few sinusoids, they can be accurately determined.

Alternatively, you could have a look at the MUSIC algorithm. I'm not too sure about the details of how it is implemented, but it has been implemented in several real-time detection scenarios. An alternative to MUSIC is the Esprit algorithm. 

Still, if your sample size is large enough and the noise doesn't swamp the signal entirely, then an FFT will be (on average) the fastest option.