bring a box out and put a blanket in it and see if she'll takes to it... rub her if she'll let you but don't handle the kittens unless you think they're in danger (dog nearby that could get to them sort of danger)
Does she look cared for? A stray who's just had kittens will usually look quite thin and haggard, if her coat looks shiny and she's well fed she's most likely someone's pet who got caught short... she won't have gone far from home whilst pregnant so knock on neighbours doors and ask.