Oh, and btw, if someone really wants to check reshiram/zekrom's nature, BEFORE the game decides to save (you're taken on an unstoppable "roller coaster" series of events that you have no control. Therefore you never get a chance to have a breather and check your legends nature before the game forces you to save and launches into the end credits. In addition, during a battle, all they display for a pokemon is the stats. No nature.)
1. Go to pokemon.marriland.com
2. click "pokedex" under Black and White on the left side
3. search zekrom/reshiram in the search box
4. scroll to the very bottom of the page
5. select the nature that you want for your legendary (I'll just pretend that the nature you wanted is modest)
6. since modest increases sp. atk, look under the column "252 EVs, and under that, Max" (the column that has some numbers that are underlined with a dotted line).
7. look on the sp. atk. row and identify the number that corresponds with both the column and row.
8. hover your cursor over that number, and a number will pop up (exceptions with the HP stat.)
9. record that number (for the HP stat, just copy that number down)
10. If you don't care about IVs, then subtract 31 from the number that you recorded (in other words, recorded #-31=) and record the resulting number with the first recorded number. (same with the HP stat). The first recorded number is the number that appears if your legendary has a perfect 31 IVs. The second is the number that appears if your legendary has 0 IVs. If the stat that you want is within those numbers, then it has at least a nature that increases the stat that you want
11. perform the same procedure from 6-10 for the decreased stat, except that you look under the columns "0 EVs" and just record the "min" and "max" numbers on the row of the stat that you want decreased, except that there's no cursor hovering.
12. Catch your legendary and if it's stats fit your requirements, then congrats! you got your legendary!
tip: get a synchronizer, such as munna, musharna, elgyem, and beeheyem. either faint them and put them in front of your party or just send it in first and switch out right after. There's a 1/6th chance that your legend's nature is going to be the same as your synchronizer.