Circuit to Detect Floating Signal Line
If the ADC is part of a typical microcontroller (MSP430, the AVR in an Arduino for example) you will probably find there are optional pullup and pulldown resistors with high values (50kilohms for example). 

If you enable the pullup only, read the ADC, enable the pulldown only, read the ADC again, and the two readings are very different, you can reasonably conclude the signal is floating without any additional hardware. 

[EDIT] assuming the ADC is monitoring a slow moving voltage. If it is monitoring high amplitude noise, a single pair of readings may not be enough to make the decision. Nevertheless the pullup/pulldown resistors should pull the signal close to the appropriate rail.