A stack master retains its role unless one of these events occurs:
The switch stack is reset.*
The stack master is removed from the switch stack.
The stack master is reset or powered off.
The stack master fails.
The switch stack membership is increased by adding powered-on standalone switches or switch stacks.* In the events marked by an asterisk (*), the current stack master might be re-elected based on the listed factors.
When you power on or reset an entire switch stack, some stack members might not participate in the stack master election. Stack members that are powered on within the same 20-second time frame participate in the stack master election and have a chance to become the stack master.
Stack members that are powered on after the 20-second time frame do not participate in this initial election and become stack members. All stack members participate in re-elections. For all powering considerations that affect stack-master elections, see the "Switch Installation" chapter in the hardware installation guide.
The new stack master becomes available after a few seconds. In the meantime, the switch stack uses the forwarding tables in memory to minimize network disruption. The physical interfaces on the other available stack members are not affected during a new stack master election and reset.
After a new stack master is elected and the previous stack master becomes available, the previous stack master does not resume its role as stack master.
As described in the hardware installation guide, you can use the Master LED on the switch to see if the switch is the stack master.
https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst3750x_3560x/software/release/12-
2_55_se/configuration/guide/3750xscg/swstack.html