Assume the wheel is dished correctly for its current spacing.
The new dish will be off by 1/2 of the amount of spacing you add.
It's possible the dish has "migrated" in your favor to some extent and you end up very close to correct.
It seems a lot of generic, machine built rear wheels shift that way as they settle in.
You can only tell by measuring.
Flipping the wheel in the drop outs and see if the rim is the same distance from some convenient ref. point on the chain stay will get you quite close.