Debye polarization, ionic displacement polarization, and Maxwell-Wagner (interfacial) polarization are discussed in this paper, because they would most likely take place in an electrorheological (ER) suspension. The temperature dependences of the dielectric loss tangent maximums governed by these three types of polarization are theoretically found to be quite different. Given this fact, a method that can directly distinguish the polarization type and clarify which polarization would be mainly responsible for the ER effect is proposed. Two kinds of typical ER suspensions, heterogeneous particle type and homogeneous liquid crystalline polymer type, are studied using our method. It is found that Maxwell-Wagner polarization would be responsible for the ER effect both in a heterogeneous and in a homogeneous ER system. These findings present direct experimental evidence for the previous assumption that the Maxwell-Wagner polarization would dominate in the heterogeneous ER system and also shed light on the ER mechanism in a liquid crystalline polymer-type ER system.