HADAA, it's fine to reivive an old thread as long as your post contributes to the topic in a way that will generate more discussion (and as long as the thread isn't ultra-old).
Anyway, HADAA's method is probably best for knowing exactly what nature your Reshiram / Zekrom is.
While I was thinking over other possible methods when I was catching Reshiram, I came up with something. Use an IV calculator to calculate the IVs of Reshiram / Zekrom at Level 50 with the nature you want. (IVs should be set to 31, of course.) When you catch Reshiram / Zekrom, check its stats. If its stats are within the range of stats you got with the calculator, it may be the nature you want.
For example, let's say you're aiming to get a Modest Reshiram. A Modest Reshiram at Lv.50 with max IVs will have a Special Attack stat of 187. With a neutral nature, the highest its Sp. Attack can be is 170. If you catch Reshiram and its Sp. Attack is higher than 170, it has a Sp. Attack boosting Nature. Using this method, you can also kind of figure out what its nature lowers by checking the maximum of a lowering nature and the minimum of a neutral nature, though it may be hard to pinpoint since the numbers may vary with low IVs.
And, since you're checking its stats already, you're making sure the Reshiram you're catching has good IVs!
This isn't a sure-fire thing, but it may be quicker than losing to N as you can just soft reset as soon as you check the stats.
