You're very correct. The differentiation likely takes years of training to fully develop. A general rule of thumb to keep injury risk low is to never push yourself more than 10% of what you normally do (lots exercise books say this). If you can hold that level for several weeks without problem, it's likely safe to "notch up" another 5% or 10%. Lots of other variables at play too, of course (rest, stretching, warmup, etc...).