My personal experience with this-
I think it takes A LOT to actually burn your CNS and get anything close to genuine exhaustion. I cannot imagine any actual beginner burning out just by doing starting strength/other beginner programs or some form of exercise 3x week. What most beginners say about being tired is just them being unable to move out of their comfort zone.
I restarted lifting back in March and did the following routine-
Push-up variations. Around 50 reps total.
Dumbbell OHP. 3x10 at 20lb. 3x8 25lb, 3x5 working sets.
Walking for a whee bit.
Dumbbell rows/Good mornings 3x5. Alternating.
Max incline treadmill runs as outlined by Jim Wendler in his conditioning article. Started at a speed of 8 and 12 sets total. Added another 2 sets every lifting day till 20. Stay at 20 for 1-2 days. Then I increased the speed by .3 and restarted at 12 sets and vice versa. Started doing this mid-April.
Spent a large part of March/April really just spinning my wheels and trying to get my squat and deadlift form back. Evidently 4-5 months off is sufficient for totally killing my muscles and breaking my ability to squat to all hell. Makes me worried for the expected two month hiatus I need to take for my wrist surgery.. Really got back into making gains mid-April/early-May.
Got to roughly the following in terms of numbers-
Squat- 210x5, tested 1rm of 235.
Deadlift- 300x5, tested 1rm of 345. (Grip slipping damn near ruined me though. I really need to start using chalk..)
Then, last week, I just felt dead. Did my exercises on Tues and the squats felt like pure torture. Afterwards I was limping. Went back on Thurs and utterly failed my squats. Running went well though. Went back on Sat and did deadlifts fine, but the squats just didn't happen. My form was completely out of whack, and my hip felt tighter than they should be. Got to my last warm-up set of 185 and I gave up and left the gym.
Now, I've felt lazy before. I feel lazy all the time on my push-up days. They're boring and I just do them mainly to do something with my upper body. I've had days when I didn't want to go to the gym; but I've never felt as drained as I did last Saturday.
But I wouldn't have known whether I was actually reaching a point of genuine fatigue unless I got there. I had no idea I was tired until the entire week passed. So, the only real way afaik is to just go to the gym and try your normal routine. If it happens, then it happens. If it doesn't, then try it again for the next couple of days. If things get progressively worse, then you should probably stop and take a deload week or something.
And CNS burning seems to be localized. My squats suffered horribly while everything else was bright and perky. I honestly have no idea whether my squats failing in the past week was due to actual fatigue or just me pussying out after how painful Monday was.