Convergence is not always a simple thing to confirm. Obviously residual errors could give a hint but not always. In some steady-state simulation, you need to check the change in forces with every iteration. This is one example. Another example is when your simulation never reaches the desired residual error and here you should ask whether your simulation might be transient in nature?. In Fluent the main indicators are the residual errors for the turbulence equations, the momentum and continuity equations.