To probe magnetic ordering in single crystals of La1-xMnO 3 (x = 0.01, 0.05, 0.13) and in ceramics of LaMn1-yO 3 (y = 0, 0.02, 0.06), the X-band electron magnetic resonance measurements were carried out in the temperature range 5 K ≤ T ≤ 600 K. The temperature dependences of doubly integrated intensity of electron paramagnetic resonance signal and its linewidth were fitted with known theoretical models modified for taking into account the different mechanisms of spin relaxation. Both experimental data and fitting results evidence that vacancies in Mn- and La-sites dope the carriers, which induces ferromagnetic double exchange interaction in parent LaMnO3. However, strong structural and chemical disorder of La1-xMnO3 crystals makes the ferromagnetic ground state eventually impossible even at x = 0.13. In marked contrast, better structural/chemical homogeneity together with a stronger impact of Mn-site vacancies on mixed manganese valence and double exchange are characteristic for LaMn1-yO3 ceramics. As a result, the LaMn0.94O3 compound appears to be ferromagnetic-like ordered and demonstrates band-like character of the doped carriers. It is shown that 'self-doped' LaMnO3 may be considered as a model system for studying both transition from antiferromagnetic to ferromagnetic-like magnetic ground state in La manganites upon doping and the influence of structural/chemical disorder on such a transition.