You need to measure methane production over time. From the resulting curve you can evaluate whether you have read the maximum amount of methane or can expect more.

Clearly this is a vague answer. It is possible that your substrate consists of different fractions and that these fractions are degraded sequentially. In that case, even if methane production has reached a plateau it is still possible that methane production will resume when a less degradable fraction also starts to be degraded.

Thus it is useful to measure COD of your substrate. With this info you can easily estimate how much methane you can ultimately expect, at least in theory.

One method is to measure the elementary composition of your VS and calculate the possible methane production using Bushwell equation, which gives the theoretical methane potential and assumes complete degradation of your VS. Another methods is to do batch experiment using your waste. This gives the biochemical methane potential (BMP), but the value will be affected by your experimental conditions such as the inoculum, total solids content, C/N ratio and so on. Hopefully this helps!

Completely agree with both answers.

Batch experiments and following  methane production will give you the answer in YOUR experimental conditions.