For example, see this Wikipedia section on Newton's laws of motion:
Newton's Second Law states that an applied force, $\mathbf F$, on an object equals the rate of change of its momentum, $\mathbf p$, with time. Mathematically, this is expressed as: $$\mathbf F =\frac{\mathrm d \mathbf p}{\mathrm dt} =\frac{\mathrm d(m \mathbf v)}{\mathrm dt}.$$
Based on this chart of symbols I understand they are saying $A / B = C / D$ and that $\mathrm d$ is a function and $m \mathbf v$ is the input to that function, but how do I figure out what "$\mathrm d \mathbf p$", "$\mathrm dt$", "$\mathrm d$", and "$m \mathbf v$" mean? Please don't give me the answer, but instead, pretty please tell me what method a math person would use to always know what there mean (without asking anyone).
I can sort of guess that $m \mathbf v$ might mean motion/velocity or something, but surely you're not just supposed to guess at the symbols. A see formulas like this all the time and I can never find a key that explains what the letters mean. What am I missing?