Not necessarily in order, sometimes you have to trust your instinct on that (although when I try, I usually wind up wrong(sigh):
- Length of line
- Estimated speed of Cashier - some of the ones at my grocery store are just horribly slow, others are fast.
- Amount of stuff in others' carts/baskets
- Coupon watch - folks with a fistful of coupons always seem to take forever
- Checkbook watch - enough said.
- Age of fellow shoppers (ie, little old lady watch... but I've been burned on this watching some darned efficient little old ladies blast through leaving me in the wrong line, too)
- Number of children in tow (with extra "stay away" modifier for poorly behaved/tired/whining/"mommy buy me candy":"NO!" children)
- Is it an express 20-items or less line?
- Is it the self-checkout line?
So, a lot I guess .