Find out at any tie if weeds are dead, simply by lifting the polythene. If you see white/yellow (because light-deprived) stems, there are still weeds trying to grow from resources in their roots.
From July, you may now see almost nothing, it depends what was there.
I would not try and plant through it now, there will be slugs through winter.
Best wait until spring as I say above, remove the cover completely and start over.
