how to know if a device has a camera with flash?

You can do it by:

getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH);

What you do is check the system features if the camera supports the flash CAMERA_FLASH

This would be the Toast :

Toast.makeText(getApplicationContext(), getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH)?"Camara tiene Flash":"Camara no tiene flash",Toast.LENGTH_SHORT).show();

To check flash availability on a device:

Use:

context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH);

Returns true if the flash is available, false if not.

Very good. You can check if you have flash as follows:

boolean hasFlash = this.getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA_FLASH);

If this code does not work for you, you can try this one:

Camera camera; // Inicializas la cámara
public boolean hasFlash() {
    if (camera == null) {
        return false;
    }

    Camera.Parameters parameters = camera.getParameters();

    if (parameters.getFlashMode() == null) {
        return false;
    }

    List<String> supportedFlashModes = parameters.getSupportedFlashModes();
    if (supportedFlashModes == null || supportedFlashModes.isEmpty() || supportedFlashModes.size() == 1 && supportedFlashModes.get(0).equals(Camera.Parameters.FLASH_MODE_OFF)) {
        return false;
    }

    return true;
}

You can see the source here: link

First of all, remember that most devices have more than one camera.

What you should do is first get a list of available cameras and then search among them which one has a flash, if any has a flash then you return a 'True'.

The correct way is to do that from the Google Camera2 API.

Try this code:

public boolean hasFlash() {
    boolean hasFlash = false;
    //Primero obtienes el servicio de camara.
    CameraManager cameraManager = (CameraManager) getSystemService(CAMERA_SERVICE);
    try {
        //Obtienes todos los ID'S de las camaras disponibles
        //Por lo regular son dos (Camara trasera, Camara frontal)
        String[] cameraIds = cameraManager.getCameraIdList();
        //Buscas entre todas ellos si esque alguna tiene flash.
        for (String cameraId :
                cameraIds) {
            CameraCharacteristics characteristics = cameraManager.getCameraCharacteristics(cameraId);
            if (characteristics.get(CameraCharacteristics.FLASH_INFO_AVAILABLE).booleanValue())
                hasFlash = true;
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return hasFlash;
}