The small ones you can usualy eyeball. The point is they dont want a sharp, 90 degree corner. An easy way to check is to use drill bits. If the call out is lets say a 1/4" radius, grab a 1/2" diameter drill and hold it up to your corner. If you want to actualy draw it out on something, use a 1/2" washer and trace the inside. Remember, radius is from the center of the circle to the edge so for a 1/4" rad use a 1/2 drill, 3/32" rad use a 3/16" drill, etc. Or just go buy a set of radius or fillet gauges from a tool supplier.