Objects give off different wavelengths and amounts of radiation based on their temperature which is called black-body radiation. The radiation is given off in a continuous spectrum in a curve with a peak at a certain frequency depending on the temperature. There's a cool interactive visualization published by the University of Colorado that lets you play with parameters. Unfortunately it only lets you go down to 200K, but fortunately they made the source code available. If you play around with their simulation:
* the sun is at 5850K, gives off 84 million watts per square meter with a peak of 500 nanometers which is in the visible range
* a body at 300K gives off only 459 watts per square meter with a peak at 9,659 nanometers which is in the infrared
* at 77K (modifying source required) it only gives off 2 watts per square meter with a peak at 37,633 nanometers which is also in the infrared, need to get to about 1,000,000 nanometers to push it into microwaves
So enemies could detect you, but you're giving off 1/200th the energy of something at 300K and 1/45th the energy of something at 200K (typical asteroid), so you would be that much harder to detect. Also their equipment might need to be tuned for different frequencies or they may have to have different equipment all together to detect you compared to a normal ship. If stealth ships are a new technology, they might not be prepared and miss you all together.
But you have to think about where the energy from your goes. If you have a layer of vacuum between your actual hull and the liquid nitrogen, you'll cook inside if you're producing much energy at all. 184
If you dump heat into the liquid nitrogen it will boil and increase the pressure until your container bursts. If you're near the sun and don't reflect it's radiation, that will warm it up too (ala 200K in the asteroid belt) It might be efficient to have blocks of regular water ice for this purpose. 1kg of ice needs to absorb about 330kJ of energy to melt, then it will take another 420kJ to raise it to the boiling temperature. So you can have a certain energy budget you can maintain your stealth mode for before you need to vent some of that heat. If enemies are only in one direction, you could have heat pipes transferring that heat to a large radiator that could emit heat away from your enemies which would be nearly impossible to detect from the opposite direction.
You might be able to rendezvous with a comet or asteroid and dump some heat into that. You would need some form of propulsion that would let you perform a rendezvous without being seen though, like some form of reactionless drive. Even if you vent the liquid nitrogen for thrust, it will expand to make a larger cloud and will be warmed by the sun which will both serve to increase your energy signature and provide a way to track your trajectory. If you start lighting off a fusion drive, you can forget about stealth. 184