The easiest test is to disconnect the battery, press the power button to clear the SMC latch state. Now plug in the battery and the MagSafe. Now when you restart your system just as you here the gong press the T key to enter into Target disk mode!
Reference: Mac startup key combinations
At this point your display should be showing the FireWire or Thunderbolt Icon. If it does you’ve proved the Intel graphics engine is working! Now if you restart your system again your AMD GPU will kick in at the end of the POST cycle and your system will either go to a black or white screen or in some cases colorful blocks/bands.
There are a few different ways to lock out the AMD GPU so you’re left with the Intel graphics only. But you’re stuck with nothing higher than Sierra
Edit the NVRAM variable from Single-User mode or use fxCardStatus to use the intergraded graphics. But if you loose the setting you’ll need to start again.
You can also use this DeMux from DosDude this will allow you to go upto High Sierra.
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