can I weigh booster packs to find rares
can I weigh booster packs to find rares
no.
Rare cards are not any heavier than any other card, they're all printed on the same paper with the same ink and everything. You can however compare weights of packs in your hand to find foils, as they are slightly heavier. Not everybody can differentiate though.
On another note, every single pack has one rare or mythic. So why would you want to find out if a pack had a rare? They all do.
And even if you did weigh to find a foil, you would still have no idea which foil it would be, from a crappy common to the top mythic. So in short, just buy 'em, crack 'em and play.
If you're looking for specific cards, you can buy them online. Though unless you play EDH or competitive constructed, don't bother. It can be a lot of money if you don't know the right sites. If you play casually, grab a couple intro packs and a few boosters and just have fun. Or go to drafts/sealed events in your area, great way to have fun and get new cards.