How do we figure out whether reactions are endothermic or exothermic?
If you search for &quot;standard enthalpy of formation hydrogen iodide&quot;, you get positive value $\pu{26.5 kJ/mol}$, what means the reaction is endothermic, as by convention, the values for elements in their standard state is zero.
The other one is strongly endothermic, what you realize, if you search the same for nitric oxide.
For a general reaction, search for standard enthalpy of formation of all reactants and products, multiply the values by stoichiometric coefficients and subtract the reactant values from the product values. If the result is negative, the reaction is exothermic. For the positive value, the reaction is endothermic.
For starters: enthalpy is defined as $H = U + p . V$, where $U$ is system internal energy, $p$ is pressure, $V$ is volume. Change of system enthalpy in constant pressure condition is change of system internal energy + volume work done by system: $\Delta H = \Delta U + p . \Delta V$. By other words, $\Delta H$ is the energy you have to provide to a system at constant pressure to increase its internal energy by $\Delta U$, if the system changes its volume by $\Delta V$.
For guessing the formation enthalpy patterns, there is no easy shortcut. For simple inorganic compounds, a lot of help in noticing patterns is the periodic element table.
All comes with experience and learnt expirical knowledge and patterns. Chemistry is in large extent empirical science, where many things cannot be derived. Or can, but it is much easier to just remember, unless one wants to jump in quantum chemistry. Even then not everything can be calculated.