Overtraining is very REAL, I know your new to the forums OP but I strongly suggest you ignore training advice from people who claim it doesn't exist.
If it didnt exist why dont people get stronger in a linear fashion? Anyone who has trained naturally knows that this doesn't happen and its not simply down to caloric intake. Those who ignore overtraining are generally AAS users because AAS use compensates for overtraining.
Back in the day the likes of Arnold could train twice a day with high intensity and get away with it BECAUSE they were on cycle, training like this is NOT optimal whatsoever but the AAS use will cover you for the time being.
There are also countless scientific studies showing that overtraining exists with elite athletes, in fact this is one of the main reasons high level coaches use periodization to create workout programmes.
As for signs of overtraining op, if your progressively getting stronger (lifting more weight) then your fine. If your stagnating for a long time or getting weaker then I suggest a couple of deload weeks or a complete week off to help your body recover.