Rattlesnakes don't have eyesight that is great, but they can see and detect movement. They can certainly feel the vibrations of your movements through the ground and taste you on the air with their tongue. In addition, they can sense you via infrared (heat) detecting organs located in pits in front of their eyes. (The rattlesnake is a pit viper, and it gets that name from the "pits" which are the openings to the heat sensing organs.) A link is provided to a picture of a pit viper on Wikipedia. The pits are clearly visible.
heat-sensitive organs
They can detect both visible light and infrared radiation.
The rattlesnake can detect temperature changes of 0.003 °C or less in its immediate surroundings.
to detect the presence of liquid nerve and blister agents to detect the presence of liquid nerve and blister agents
You can detect the presence of a magnetic field using a compass, which will align itself with the field lines. Other devices such as magnetometers can also detect and measure magnetic fields accurately. Additionally, certain materials, such as iron filings, can visualize magnetic field patterns when sprinkled around the area.
Taste it
To detect the presence of CI- ion, add silver nitrate solution. A white opalescence to cloudy precipitate of silver chloride will form and confirm the presence of CI- ion.
an electroscope
The presence of halide ions
PH test
To detect the presence of CI- ion, add silver nitrate solution. A white opalescence to cloudy precipitate of silver chloride will form and confirm the presence of CI- ion.
Sudan III is used to detect the presence of lipids.
a multimeter?