Set a pin HIGH when crash occurs
The ESP provides a reset reason API, which lets you differentiate between a crash and a normal reset:

extern "C" {
  #include "user_interface.h"
}

const char * const RST_REASONS[] =
{
  "REASON_DEFAULT_RST",
  "REASON_WDT_RST",
  "REASON_EXCEPTION_RST",
  "REASON_SOFT_WDT_RST",
  "REASON_SOFT_RESTART",
  "REASON_DEEP_SLEEP_AWAKE",
  "REASON_EXT_SYS_RST"
};

// .. in setup():

const rst_info * resetInfo = system_get_rst_info(); //populate reset info
String diag = String(RST_REASONS[resetInfo-&gt;reason]); // turn enum index to string

if(diag == "REASON_SOFT_WDT_RST") digitalWrite(TX, HIGH);// check string, take action