I have some grass that I want to clear out to make way for a garden. I covered it in black plastic and have waited for a number of weeks. How do I know when the grass is dead? See below for a photo of the current state. And what do I need to do to adequately clear it out for the garden?
Leave it exposed to the light and pour a bucket of water on it.
If it's green again within a few days, it wasn't dead.
I'm fairly sure I can still see some green grass in both your pictures so that is certainly not dead yet. "A few weeks" doesn't sound long enough to me. Grass doesn't die easily. Between 6 months and 2 years would probably be long enough.
Research how to make compost, the heat of a compost pile will kill the seeds and "kill the grass".
The state it is in now - it will continue to grow because there are seeds in there, and those can wait for perfect conditions to grow for years. As soon as there is enough moisture and heat to germinate, it will grow.
In compost there will be no alive seeds, but the wind will bring them in with the 1st breeze, can't fight nature. Comes down to what is the purpose of this and what do you want to achieve.