Identifying electron donors vs electron withdrawers (6C 21)

[Nathan Nakaguchi 1G]

I understand that electron withdrawers would make a molecule more likely to lose a proton and electron donors less likely to lose a proton. Like in 6C 21 how acetic acid is a weaker acid than formic acid since the methyl group -CH3 on the acetic acid is a electron donor to the molecule making it slightly less likely to give off a proton than formic. How can one identify when a molecule has either electron donors or electron withdrawers with it? How do you know that -CH3 is a electron donor while something like -CCl3 is an electron withdrawer?

[Sanjana Borle 2K]

Basically electronegativity determines the electron withdrawing ability of atoms in a molecule, and therefore its acidity. So for the CH3 vs CCl3 example, since Cl is more electronegative, the Cl atoms pull the e- density closer to themselves, making the H easier to detach.