When shown diagrams of isomers, how can you determine which is a structural and which is a geometric isomer?

The word $\text{connectivity}$ is key. Consider $\text{2-butylene}$, which structure can generate both cis and trans geometric isomers. For both geometries the connectivity is the same: $\text{C1 connects to C2, which connects to...C4}$. Nevertheless, the cis and trans isomers are manifestly different with respect to geometry, and are thus geometric isomers, or even diastereomers. (Note, here I use the definition that diastereomers are geometric isomers that are not mirror images.)