How do we know chaotic systems are actually chaotic and not periodic?

Well, on the one hand, yes, chaos is a mathematical abstraction so, for instance, there will never be an experimentally or numerically measured Lyapunov exponent, only finite Lyapunov exponents (FLEs) - the same way there won't ever be a sphere, or any exponential growth (the universe seems finite), etc. They are idealized constructs that only approximate the physical reality. On the other hand, these idealizations have been proven very useful inummerous times.

As for

who's to say it wouldn't converge and repeat itself after a really long finite time?

there are rigorous mathematical proofs for some systems, and we can consider a simple one such as $$ x_{n+1} = 10\cdot x_n \mod 1, $$ which, by multiplying by $10$ and truncating to bellow $1$, produces trajectories such as: $$ 0.123456 \to 0.23456 \to 0.3456 \to 0.456 \to \ldots$$ which, for almost all$^1$ initial points, leads to infinite, never-repeating trajectories. And arbitrarily close initial conditions diverge exponentially fast: actually, if two random initial conditions coincide for the first 40 digits only, after $n=40$ iterations of the map above, their trajectories bear no relation to each other any more. These are certainly chaotic trajectories.

$^1$ This is a rigorous statement, in the sense that the irrational numbers have measure 1 on $[0,1)$.

A mathematical model can be chaotic. A real life system can exhibit chaotic behaviors. That minor word shift helps capture the idea that a system is unpredictable in the same way that a chaotic system is, but which may not be completely chaotic. This is rather an important detail because no system exists in a vacuum. All real life systems interact with their environment, and this interaction can make it hard to tell whether the unpredictability is coming from inside the system or outside.

The systems you refer to which might converge at a later date are in a region known as The Edge of Chaos. This is the fuzzy boundary between order and chaos that is even more frustratingly complex than chaos itself. Evolution is a famous example of a process that is on the edge of chaos. It is not clear if it will converge to order. It is not clear if it will spiral off into chaos. Indeed, it isn't even clear that it has to do one or the other!