Identify Previously Registered Users (Android/Java)

I was wondering if anyone could point me in the right direction with this. I have an SQL Lite database set up that allows the user to add their account details. It also allows to edit the account details when returning to the application.

I really want the application to identify whether the user has previously added an account or is a new user. If they are a new user it will display a register button. If it is a returning user it will forward them onto the main menu activity.

I will attach some code as an example.

Any help would be greatly appreciated!

Register.java

   import android.content.Intent;
   import android.support.v7.app.AppCompatActivity;
   import android.os.Bundle;
   import android.view.View;
   import android.widget.EditText;
   import android.widget.Toast;

   public class Register extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_register);
}

public void btnCreate(View view) {

    SQLHelper sqlHelper = new SQLHelper(this);

    EditText addName = (EditText) findViewById(R.id.nameText);
    EditText edtEmail = (EditText) findViewById(R.id.nameEmail);
    EditText edtNumber = (EditText) findViewById(R.id.editNumber);
    EditText edtAge = (EditText) findViewById(R.id.editAge);
    EditText edtNurse = (EditText) findViewById(R.id.editNurse);

    sqlHelper.insert(addName.getText().toString(), edtEmail.getText().toString(), Integer.parseInt(edtNumber.getText().toString()), Integer.parseInt(edtAge.getText().toString()), edtNurse.getText().toString());
    Toast.makeText(this, "Account Has Been Created", Toast.LENGTH_SHORT).show();
    startActivity(new Intent(Register.this, MainMenu.class));
}
   }

Welcome.java:

   package my.example.com.mymedicare;

   import android.content.Intent;
   import android.support.v7.app.AppCompatActivity;
   import android.os.Bundle;
   import android.view.View;

   public class Welcome extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_welcome);
    };


public void btnRegister(View view) {

    startActivity(new Intent(Welcome.this, Register.class));
}

public void btnContinue(View view) {

    startActivity(new Intent(Welcome.this, MainMenu.class));
}


   }