how would you know if a compound would go SN2 or E2 ?

Question

also in terms of a strong base for E2 vs a strong nucleuophile for SN2 .. and in terms of having a tertiary>secondery>primary carbcation.

Answer 1

You've just about answered it yourself.
The following conditions favour elimination:
strong base, non-aqueous conditions (ie in ethanol), high temp, tertiary halogenoalkane.

The following conditions favour substitution:
milder base (or nucleophile) aqueous, moderate temp, primary halogenoalkane.