What size tire using 15x6 wheel?
Offset can absolutely be measured. It's just the distance of the mounting surface from the centerline of the rim. The problem is that people don't understand that backspace is measure from the outside lip of the wheel, so when they try to convert backspace to offset they forget the extra ~1/2" of rim lip included in the backspace.
So, for example, a 15x8" with a 4.5" backspace. That's a 0mm offset, but people think that's wrong because the backspace is 4.5" not 4". Well, the 8" width listed in the spec is where the tire bead sits, not where the outside lip of the rim is. A 15x8" is actually 9" wide if you measure outside to outside, which is where the 4.5" backspace comes from. So the centerline is at 4.5" from the outside, which means the mounting surface is at the centerline.
And offset can be a very helpful specification. If you're changing the width of the rim, for example, you can just get a rim with the same offset. The additional width will be split between the inside and outside, if the offset is the same the centerline of the wheel is the same. If you keep the backspace the same and use a wider rim, all of the additional width is on the outside.
Another example- an 18x9 with a 6" backspace. If you remember that the 18x9" is actually 10" wide outside to outside, then you know that the centerline of the wheel is at 5". So if it has a 6" backspace, then it has a 1" offset, which makes is a +25mm offset (well, 25.4mm but it will be rounded down). Offset is always listed in mm, which also confuses folks.
Housing width is a lot easier to measure though because it's usually measured on a bare housing outside the car so there's nothing in the way and the tape measure will lay flat across the axle housing. Wheel mount-to-wheel mount on a fully assembled axle usually requires a couple of straight edges and an extra person to measure accurately. As long as you know what brakes are used you can calculate the WM-WM if you have the housing width.
Offset can absolutely be measured. It's just the distance of the mounting surface from the centerline of the rim. The problem is that people don't understand that backspace is measure from the outside lip of the wheel, so when they try to convert backspace to offset they forget the extra ~1/2" of rim lip included in the backspace.
So, for example, a 15x8" with a 4.5" backspace. That's a 0mm offset, but people think that's wrong because the backspace is 4.5" not 4". Well, the 8" width listed in the spec is where the tire bead sits, not where the outside lip of the rim is. A 15x8" is actually 9" wide if you measure outside to outside, which is where the 4.5" backspace comes from. So the centerline is at 4.5" from the outside, which means the mounting surface is at the centerline.
And offset can be a very helpful specification. If you're changing the width of the rim, for example, you can just get a rim with the same offset. The additional width will be split between the inside and outside, if the offset is the same the centerline of the wheel is the same. If you keep the backspace the same and use a wider rim, all of the additional width is on the outside.
Another example- an 18x9 with a 6" backspace. If you remember that the 18x9" is actually 10" wide outside to outside, then you know that the centerline of the wheel is at 5". So if it has a 6" backspace, then it has a 1" offset, which makes is a +25mm offset (well, 25.4mm but it will be rounded down). Offset is always listed in mm, which also confuses folks.
Housing width is a lot easier to measure though because it's usually measured on a bare housing outside the car so there's nothing in the way and the tape measure will lay flat across the axle housing. Wheel mount-to-wheel mount on a fully assembled axle usually requires a couple of straight edges and an extra person to measure accurately. As long as you know what brakes are used you can calculate the WM-WM if you have the housing width.