How do I verify polarity with ultra low voltage?

I have a voltage source of 20 mv to 120 mv and I want to make sure that I always bias my sensing device correctly + to +, and - to -. Although it does not matter to my device, I just want to keep orientation of my + and - wires, as they designate a physical position of an item at the other end. I tried using a diode, but the voltage is too small for any diodes that I have. Can anyone suggest another component or set of components to verify this ultra low voltage polarity.
Thanks....Dom

I assume you are wanting to use something that requires no external power source ... a tough requirement.

I have not used these but I would look for devices intended to be used in energy harvesting applications, such as http://www.aldinc.com/pdf/EH4295.pdf This device claims 60mV typical startup voltage, but does not say what happens if the input voltage is reversed - maybe check with ALD on that.

You don't mention how much power is available from your source. Depending on that and the harvester outout, you can come up with an indicator. One thought is a simple LCD pane element which darkens when voltage is present and is clear when not. I got one as a sample from a company who made custom LCD displays, but don't recall who it was.

Hello Todd and thanks so much for your help with my situation. To be fair, I should tell you exactly what I am doing. I have a requirement to determine whether a remote object is facing up or facing downward. This object looks exactly the same on the outside, whether facing up or down, but internally the components will act differently if oriented one way or the other. I thought perhaps if I had the object fit into a holster of some sort. When the object is placed into this holster, it also makes contact with 2 terminals and the object outputs a very small amount of voltage (50mv approx.) The output would be + - if holstered one way, and - + if holstered the other way. The 2 terminals are connected to 2 wires that run to a remote location. I need to know if the object is oriented correctly (facing up). I was hoping to have something light up, if the object is orented correctly, at the remote location. If the object is not oriented correctly however, I still want to be able to verify that it is in fact holstered by pressing a DPDT momentary switch reversing the contacts and having something light up, verifing that it is holstered, but just in the wrong orientation. I hope this makes sense. Anyway, since the voltage/power is so low, I can't find a diode or an LED that will respond to this low forward voltage. I was thinking maybe a germanium diode to determine the orientation, like the ones used in crystal radios. You have given me a fantastic idea using the lcd panel as the object to be lighted . I am just wondering what happens to the panel when biased in the wrong direction and what is the minimum voltage is to operate one of these. I will definitely look into this. Thanks again Todd....Dom

Could you have a battery powered device on this "holster" ? And/or a solar cell to charge or power it ?
If so, a comparator IC based circuit would be a simple device to detect a very small voltage polarity and provide an output showing + or -.

With a magnet fitted to the object, another powered circuit possibility is a Hall effect sensor which would provide a different ouptput depending on which pole of the magnet is presented to it.

Quote from the Hall Effect Ratiometric Sensor A1301/A1302 datasheet:
"Sensitivity. The presence of a south-polarity (+B) magnetic field, perpendicular to the branded face of the device package, increases the output voltage, VOUT, in proportion to the magnetic field applied, from VOUTQ toward the VCC rail. Conversely, the application of a north polarity (–B) magnetic field, in the same orientation, proportionally decreases the output voltage from its quiescent value."

(I am now very curious to know what the object is !)

There isn't any other way "but" to do it as Todd procedure... you could use a LCD indicator to find if you have the right polarity...