When does a reaction equation require the reversible sign and when does it not?
You just need to learn and know. Luckily, there is a certain logic to the question which reactions can be reversible and which cannot. However, a full list is impossible and would exceed the space of this margin.

For most mechanisms, there is (at least) one point-of-no-return step. Learn to identify this step. All other arrows should probably be $\ce{&lt;=&gt;}$, if you’re strict. These non-reversible steps are often (but not always!) carbon-carbon bond forming reactions, as a very rough guideline on which is which.