I think the short answer is you probably won't until you try it.

You could start by measuring the clearance you have at your middle ring now and see if it looks like a 42 tooth ring will fit. You can estimate the radius of a 42 tooth ring by measuring your current rings and estimating where a 42 tooth ring would fall. Then measure the distance from your crank centerline (axle spindle) back towards the chain stay...at the middle ring position...to see how much distance you have there. I imagine it's going to be close and, if you're dead set on that 42 tooth ring, you may need to lengthen your axle spindle to bump your crank out just a little bit to buy you some room at the chain stay.

Because of the prevalence of so many different bike frames available, some with 68mm bottom brackets and some with 73mm bottom brackets, some designed to clear wider tires than others (which influences chain stay shape), you may need to do some trial-and-error with your crank and bottom bracket to find a combination that best suits your exact frame.