All integrals exist, of course. But the bulk of them can only be done numerically. The answer to your question is lots and lots of practice. You learn to recognize specific forms over time, those that can be done and those that can't. Certain fields of study usually have typical types of integrals and you can specialize from those. For example, in Mechanics, common undoable integrals are those of the form of elliptic integrals. Similar comments also apply to Differential Equations, which can be seen as a kind of integration.I have seen people deduce at a glance that an integrand's antiderivative does not exist in terms of elementary functions. How do they do this?