All meshes derive from IBaseMesh.  All meshes is a bit of overkill, there are 2 mesh types - static( IMesh ) and dynamic( IAnimatedMesh ).

Here's sample code that shows how to load a mesh, detect which type of mesh it is by using getMeshType, and the load that mesh into the appropriate Scene Node for rendering by the Scene Manager.

scene::IBaseMesh* mesh = smgr->getMesh( "mesh.x" );
if( mesh )
{
if( mesh->getMeshType() == scene::EMESHTYPE_STATIC )
{
scene::IMesh* sMesh = (scene::IMesh*)mesh;
sMesh->uploadMeshData( driver );
scene::IMeshSceneNode* node = smgr->addMeshSceneNode( sMesh );
}
else
{
scene::IAnimatedMesh* aMesh = (scene::IAnimatedMesh*)mesh;
aMesh->uploadMeshData( driver );
scene::IAnimatedMeshSceneNode* node = smgr->addAnimatedMeshSceneNode( aMesh );
}
}

If you'd rather use RTTI and dynamic_cast, you could do the following :

scene::IBaseMesh* mesh = smgr->getMesh( "mesh.x" );
scene::IMesh* sMesh = dynamic_cast< scene::IMesh* >( mesh );
if( sMesh != NULL )
{
sMesh->uploadMeshData( driver );
scene::IMeshSceneNode* node = smgr->addMeshSceneNode( sMesh );
}
else
{
// We can safely assume the mesh is an Animated Mesh, since that's the only other possible class available.
IAnimatedMesh* aMesh = (scene::IAnimatedMesh*)mesh;
aMesh->uploadMeshData( driver );
scene::IAnimatedMeshSceneNode* node = smgr->addAnimatedMeshSceneNode( aMesh );
}