Understanding MAXDOP behaviour for queries going parallel
How can i test/check if those queries are actually distributing the
  work across all the cores evenly or is it still the case where all of
  them might be doing all the work on one NUMA node socket with 20 cores
  while remaining 60 are idle?


I would start by looking at overall CPU utilization on the server. If CPU is relatively high then you know that you don't have the problem that you described. In that situation, you might do one or more of the following to improve throughput:


Tune your queries to use less CPU
Run fewer queries
Get more or better hardware


If CPU is relatively low then you have a different type of CPU problem. In that situation, you might do one or more of the following to improve throughput:


Run more queries
Check wait stats to see if there's something stopping queries from running or using as much CPU as you'd like


As a rule of thumb, you should not expect that if you run 10 MAXDOP 8 queries then all 80 schedulers will end up with at least one parallel worker. The algorithm for allocating worker threads just isn't that smart. Bob Dorr provides some details here. In my experience with large servers I've found that issuing more queries than I would otherwise like was essential to get all of the schedulers to have a chance of having at least one parallel worker. Parallel worker threads are by default not bound to the same NUMA node. The situation that you're describing (only one busy NUMA node) can happen if your queries on other NUMA nodes finish first.

You may find some of the following links to be helpful if you want to look into the amount of work each CPU is performing:


Overview of Windows Performance Monitor
Resource Monitor
sys.dm_os_tasks