When Maintenance Mode is enabled magneto creates a flag file in var/.maintenance.flag

Check this file

vendor/magento/framework/App/MaintenanceMode.php

function

/**
 * Sets maintenance mode "on" or "off"
 *
 * @param bool $isOn
 * @return bool
 */
public function set($isOn)
{
    if ($isOn) {
        return $this->flagDir->touch(self::FLAG_FILENAME);
    }
    if ($this->flagDir->isExist(self::FLAG_FILENAME)) {
        return $this->flagDir->delete(self::FLAG_FILENAME);
    }
    return true;
}

To Check if maintenance mode is enabled check this function

/**
 * Checks whether mode is on
 *
 * Optionally specify an IP-address to compare against the white list
 *
 * @param string $remoteAddr
 * @return bool
 */
public function isOn($remoteAddr = '')
{
    if (!$this->flagDir->isExist(self::FLAG_FILENAME)) {
        return false;
    }
    $info = $this->getAddressInfo();
    return !in_array($remoteAddr, $info);
}

When Maintenance Mode is enabled Magento creates a flag file in var/.maintenance.flag

Check this file

vendor/magento/framework/App/MaintenanceMode.php

function

/**
 * Sets maintenance mode "on" or "off"
 *
 * @param bool $isOn
 * @return bool
 */
public function set($isOn)
{
    if ($isOn) {
        return $this->flagDir->touch(self::FLAG_FILENAME);
    }
    if ($this->flagDir->isExist(self::FLAG_FILENAME)) {
        return $this->flagDir->delete(self::FLAG_FILENAME);
    }
    return true;
}

To Check if maintenance mode is enabled check this function

/**
 * Checks whether mode is on
 *
 * Optionally specify an IP-address to compare against the white list
 *
 * @param string $remoteAddr
 * @return bool
 */
public function isOn($remoteAddr = '')
{
    if (!$this->flagDir->isExist(self::FLAG_FILENAME)) {
        return false;
    }
    $info = $this->getAddressInfo();
    return !in_array($remoteAddr, $info);
}