EtherChannel balances the traffic load across the links in a channel through the reduction of part of the binary pattern that the addresses in the frame form to a numerical value that selects one of the links in the channel. EtherChannel load balancing can use MAC addresses or IP addresses, source or destination addresses, or both source and destination addresses. The mode applies to all EtherChannels that are configured on the switch.
You can find out which interface is used in the EtherChannel to forward traffic based on the load balancing method. The command for this determination is
test etherchannel load-balance interface port-channel
Some extra options might need to be added to the command.
Source from Cisco "Understanding Etherchannel load balancing"