Spaceflight hardware tends to be extensively designed. So you'll see documents that talk about the "thermal management plan" that design for specific temperatures at specific places, then quality plans to check those calculations, and test plans to confirm them further.
Air Force Space Command's SMC-S-016 "TEST REQUIREMENTS FOR LAUNCH, UPPER-STAGE AND SPACE VEHICLES" controls some of this activity on the test end. It gives some idea of what's involved:
3.22 Maximum and Minimum Model Temperature Predictions
The maximum and minimum model temperature predictions are the hottest and coldest temperatures predicted from thermal models using applicable effects of worst-case combinations of equipment operation, internal heating, vehicle orientation, solar radiation, eclipse conditions, ascent heating, descent heating, and degradation of thermal surfaces during the service life
3.23 Maximum and Minimum Predicted Temperatures
The maximum and minimum predicted temperatures (MPT) are the highest and lowest temperatures that an item can experience during its service life, including all test and operational modes. The MPT are established by adding thermal uncertainty margins to the maximum and minimum model temper- ature predictions
4.4.2 Thermal Uncertainty Margins
For the purpose of thermal uncertainty margin specification, thermal control hardware is categorized as either passive or active. Passive hardware uses a thermal uncertainty margin, whereas active hardware uses excess power as a thermal uncertainty margin. Examples of passive and active thermal control hardware for purposes of uncertainty margin are identified in Table 4.4-1.
6.2.4 Thermal Development Tests
For critical electrical and electronic units designed to operate in a vacuum environment less than 0.133 Pa (0.001 Torr), thermal mapping for known boundary conditions may be performed in the vacuum environment to verify the internal unit thermal analysis, and to provide data for thermal mathematical model correlation. Once correlated, the thermal model is used to demonstrate that criti- cal part temperature limits, consistent with reliability requirements and performance, are not exceeded.
When electrical and electronic packaging is not accomplished in accordance with known and accepted techniques relative to the interconnect subsystem, parts mounting, board sizes and thickness, number of layers, thermal coefficients of expansion, or installation method, development tests should be performed. The tests should establish confidence in the design and manufacturing processes.
Heat transport capacity tests may be required for constant and variable conductance heat pipes at the unit level to demonstrate compliance. Thermal conductance tests should be considered to verify con- ductivity across items such as vibration isolators, thermal isolators, cabling, and any other potentially significant heat conduction path.
Engineering Development Units (EDU) of cryogenic systems may be tested in a vacuum environment for early verification of system performance and margins, and assessment of parasitic heat leaks. A thermal balance test may also be conducted to demonstrate thermal control hardware and subsystem performance and to collect data for thermal model correlation.
The tests that result from this end up being complex, as they try to reproduce the entire sequence of conditions encountered: