2 q's

I have been playing at party, pretty new to online

1) How often do you check with best hand if flush/st8 draw comes on river?

2) do you ever think someone can see your cards, sometimes people call all bluffs and fold against good hands consistently, they also seem to be ahead in chips

3) do people play together, raise/reraise when one has good hand?

thx, btw luv this site