Those plus push ups, dips, overhead press & deads are the indicators for me.
Reason why I say squats and pull-ups is because your lower body is your foundation,so if you don't squat your automatically weak IMO or not as strong as you think.
And pull-ups is all bodyweight so if you can't master your own bodyweight then again you might have some strength but not as strong as you should be.
Those are the two biggest exercises to me.
Honestly weights give you fake strength,once you stop lifting you lose strength.
Calisthenics will have you strong forever.