Hi,
You can check geodatabase connection string:
FeatureClass fc = ...
var gdb = fc.GetDatastore() as Geodatabase;
string connection = gdb.GetConnectionString();
string workspaceName = connection.Split('=')[1];
string ext = Path.GetExtension(workspaceName);
if (string.Compare(ext, ".gdb", true) != 0 ) {
}
else if(string.Compare(ext, ".sde", true) != 0) {
}