When I used to use Pokesav, I always followed these guidelines to make sure they looked legit.
Effort Value (EV) total never exceeds 510. You also can't have effort values in each stat of 85. I know it totals to 510, but that's just the way it is.
Individual Values (IV) can be 31 in each stat, and I don't think there is a way for your opponent to tell, but if they do, it'll be obvious you are cheating.
Natural stats. Don't make up the stats values. For example, don't manually input 999, for each stats. What I think you're supposed to do is after inputting the correct level, EVs and IVs, click on "stats edit" and then "all max". This will adjust your Pokemon's stats to the "natural" values.
All natural moves and abilities. This is a no brainer, you shouldn't be giving your Pokemon impossible moves like Kingdra with Dynamic Punch and whatnot. The thing with dream world though is that while it will allow your Pokemon to acquire new abilities "naturally" I don't know if the dream world abilities have been "released" yet so if they haven't been, it will look illegitimate. Keep an eye out for that.
The date your acquired these Pokemon should be a feasible date. Since White came out in around 2011, you can't have the Pokedex entry say "obtained December 1, 2003" because that would be impossible.
The means in which you acquired these Pokemon should also be natural. For example, you can't have it say your Latios was hatched from an egg, or you caught your Charizard at the Pokemon League, because Latios does not hatch from eggs and you can't catch Charizard in the wild!
Ball in which they are caught. I'm not sure if this makes that big of a difference when playing online but this can really give it away in making your Pokemon look fake. If you have your Pokemon hatched, obviously it has to be caught in a Pokeball, nothing else.
OT, trainer ID. I don't know if this makes a difference or not, but make sure these match. It looks better anyway.
These were all the things I used to check when Pokesav-ing my Pokemon, so I was able to use them online and in Pokemon Battle Revolution.
Hope this helps and good luck!