The amount of thermal infrared radiation emitted by a body depends entirely on its temperature, but temperature depends on a complex of factors. In the case of an asteroid, its surface temperature depends on how much sunlight it receives, it's albedo (how much solar radiation is reflected away without contributing to raising its temperature), and it's thermal inertia (TI). The latter is a property of the material it is made up of. Some substances take a long time to heat up, but then they take a long time to cool off, while others heat up and cool off quickly. Since most asteroids exhibit rotation of some sort their surfaces tend to heat up and cool off depending on when they are in shadow and when they are illuminated.

By observing the asteroids in both visual wavelengths (where the sun puts out most of its energy) and infrared (where hot rocks radiate most of their energy), it should be possible to determine how much energy they receive from the sun and how much they emit as heat. This should then allow you to determine their albedo, which is related to how much energy they have available to turn into heat.

Back in the eighties a satellite called Heat Capacity Mapping Mission
(HCMM)was launched to make these measurements of the earth's surface to determine the TI of surface rocks, and thereby aid in geological mapping. It was placed in an orbit syncronized with the earth's rotation in such a way that it overflew each spot in its suborbital track twice a day, once at night and once during the day, at the same local solar time each day for both passes (say, 10 am and 10 pm). The daylight visual image showed the reflectivity or albedo of the rocks, and the IR sensor showed its thermal emission both day and night. From this data it was theoretically possible to calculate the thermal inertia for each pixel in the three registered images and create a TI image of the same piece of ground.

Unfortunately, it didn't work. It turned out that the data was highly skewed by surface and atmospheric moisture (water has an extremely high specific heat, and thermal IR emission is highly correlated with altitude (both H2O and CO2 are extremely efficient greenhouse gases, with CO2 dominating at higher altitudes, interfering with thermal IR emission). The result was that the processed TI images were affected as much by time-variant ground moisture and space-variant relief effects as they were by the TI of the rock and soil. But hopefully this will not be an issue with space rocks.

Now if you're looking for alien spacecraft, you will presumably be searching for bodies that generate considerable heat internally from their normal activity, but also radiate it away in order to remain in thermal equilibrium (supposedly, a desirable condition for the crew!)

The thermal signatures should be be very different, i.e., they should look much hotter than the sunlight falling on them should leave you to expect.