The OracleCI unit contains some useful functions for this. First you need to call InitOCI (if you are already connected to a database this is not necessary). Next, you can use the following global booleans to verify the OCI version:
OCI70: Boolean; // OCI 7.0 functions detected
OCI72: Boolean; // OCI 7.2 functions detected
OCI73: Boolean; // OCI 7.3 functions detected
OCI80: Boolean; // OCI 8.0 functions detected
OCI81: Boolean; // OCI 8.1 functions detected
OCI90: Boolean; // OCI 9.0 functions detected
OCI92: Boolean; // OCI 9.2 functions detected
Note that these booleans are cumulative: if OCI81 is True, OCI80 .. OCI70 are also True.