We study hidden-charm pentaquarks in the baryon-meson molecule picture within the framework of the constituent quark model, considering all the possible quark configurations for the ground state. The strong decays of the $P_c(4312)$, $P_c(4440)$, and $P_c(4457)$ resonances are investigated. It is found that pentaquark states of all quark configurations in the model can decay through the $N J/psi$ channel, but only five pentaquark states may decay dominantly in open-charm modes. The partial decay widths in the $peta_c$ channel are in the same order as in the $p J/psi$ channel for $J^P=1/2^-$ baryon-meson pentaquark states, but the $peta_c$ channel is forbidden for $J^P=3/2^-$ states. For $P_c(4440)$ and $P_c(4457)$, the $p eta_c$, $Sigma_cbar{D}$ and $Lambda_c^+bar{D}$ channels are open only for $J^P=1/2^-$ and the $Sigma_c^*bar{D}$ channel is open only for $J^P=3/2^-$ while for $P_c(4312)$, due to the mass threshold of $Sigma_cbar{D}$ and $Sigma_c^*bar{D}$ higher than 4312 MeV, the $p eta_c$ and $Lambda_c^+bar{D}$ channels are open for $J^P=1/2^-$ but no open-charm cannel open for $J^P=3/2^-$. We strongly suggest that the spin of the $P_c$ resonances may be determined in experiments by investigating the $p eta_c$ and open-charm channels.
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