The problem is, most cards don't specifically say "Can assist another player with a check". But a lot of them can.
Read the pages MikMik suggested, and look for the text "Your Check", "At your Location", or "A Check" on a card.
"A Check" can usually be used to assist a friend in any area.
And, in advance, you can't use your weapons or attack spells to help kill a monster. You can only play cards that add "buffs"; increase the outcome of the same category or trait the active player is using.
You can cheer him on, or send an ally to help, but you can't actually fight his fight for him on his turn. ;)