Read display identification information (04h)

Other Hardware Displays
2

I don't know which library you are using.

void diag_reg(const __FlashStringHelper *name, uint8_t reg, uint8_t n)
{
    uint8_t x = reg;
    Serial.print(name);
    Serial.print(" (0x");
    Serial.print(x < 0x10 ? "0" : "");
    Serial.print(x, HEX);
    Serial.print("):");
    for (int i = 0; i < n; i++) {
        uint8_t x = tft.readcommand8(reg, i);
        Serial.print(x < 0x10 ? " 0" : " ");
        Serial.print(x, HEX);
    }
    Serial.println("");
}

void diag_show(void)
{
    diag_reg(F("ILI9488_RDDID"), 0x04, 5);
    diag_reg(F("ILI9488_RDIMGFMT"), 0x0A, 1);
    diag_reg(F("ILI9488_RDMADCTL"), 0x0B, 1);
    diag_reg(F("ILI9488_RDPIXFMT"), 0x0C, 1);
    diag_reg(F("ILI9488_RDSELFDIAG"), 0x0F, 1);
    diag_reg(F("ILI9488_DFUNCTR"), 0xb6, 5);
    diag_reg(F("ILI9488_PWCTR1"), 0xC0, 3);
    diag_reg(F("ILI9488_VMCTR1"), 0xC5, 3);
    diag_reg(F("ILI9488_VMCTR2"), 0xC7, 2);
    diag_reg(F("NVM Status   "), 0xD2, 3);
    diag_reg(F("ID4          "), 0xD3, 4);
    diag_reg(F("ILI9488_RDID1"), 0xDA, 2);
    diag_reg(F("ILI9488_RDID2"), 0xDB, 2);
    diag_reg(F("ILI9488_RDID3"), 0xDC, 2);
    diag_reg(F("GAMMAP       "), 0xE0, 16);
    diag_reg(F("GAMMAN       "), 0xE1, 16);
    diag_reg(F("INTERFACE    "), 0xf6, 4);
}

Don't worry about the ILI9488 names. It was originally written for ILI9341.

I think that the Adafruit_ILI9341 library has the readcommand8(register, index) method.

David.