In this paper, the problem of the identification of the symmetry class of a given tensor is asked. Contrary to classical approaches which are based on the spectral properties of the linear operator describing the elasticity, our setting is based on the invariants of the irreducible tensors appearing in the harmonic decomposition of the elasticity tensor [19]. To that aim we first introduce a geometrical description of Ela, the space of elasticity tensors. This framework is used to derive invariant-based conditions that characterize symmetry classes. For low order symmetry classes, such conditions are given on a triplet of quadratic forms extracted from the harmonic decomposition of C, meanwhile for higher-order classes conditions are provided in terms of elements of H 4 , the higher irreducible space in the decomposition of Ela. Proceeding in such a way some well known conditions appearing in the Mehrabadi-Cowin theorem for the existence of a symmetry plane [16] are retrieved, and a set of algebraic relations on H 4 characterizing the orthotropic ([D2]), trigonal ([D3]), tetragonal ([D4]), transverse isotropic ([SO(2)]) and cubic ([O]) symmetry classes are provided. Using a genericity assumption on the elasticity tensor under study, an algorithm to identify the symmetry class of a large set of tensors is finally provided. Contents 1.5. Fixed point sets and normalizers 1.6. Linear slices 1.7. Strata dimensions 1.8. Implicit equations for closed strata 2. The isotropic stratification of a n-uple of quadratic forms 2.1. The isotropic stratification of one quadratic form 2.2. The isotropic stratification of n quadratic forms 3. The space of elasticity tensors 3.1. The harmonic decomposition of elasticity tensors 3.2. The isotropic stratification of Ela 3.3. The Cowin-Mehrabadi condition 4. The isotropic stratification of H 4 4.1. Lattice of isotropy 4.2. Global parametrization 4.3. Cubic symmetry ([O]) 4.4. Transversely isotropic symmetry ([O(2)]) 4.5. Trigonal symmetry ([D 3 ]) 4.6. Tetragonal symmetry ([D 4 ]) 4.7. Orthotropic symmetry ([D 2 ]) 4.8. Bifurcation conditions for tensors in H 4