Auto Correct and Spell Check features are enabled by default on the iPhone keyboard. To enable and turn on spell check on iPhone 6 while the auto correct is off, do as follows:
1. Open the Settings and go to the General settings
2. Scroll down to the Keyboard settings
3. Make sure the Auto-Correction option under the All Keyboards settings is Enabled (If Auto-Correction is disabled, you won't see the Spell Check option)
4. Scroll down to the English settings and toggle on/off the Check Spelling to enable/disable it on the iPhone keyboard.
5. If you have enabled the Spell Check feature, now you may go back and turn off the Auto-Correction feature. The final result is an enabled Spell Check feature and a disabled Auto Correct feature.