I'm not sure/don't remember if it's been mentioned up to now, but I guess it's worth mentioning again if it has been. If you're SRing for a neutral Speed nature, you can use the Staraptor + Speed O-Power trick, then switch to something that is one point higher in max Speed than the Pokemon you're SRing to make sure your synch worked.
e.g. I used a Natu to synch, Speed, and HP test for a Modest Latios (for reference, the Natu has 158 Speed, holding a Macho Brace it has 79). Modest Latios with neutral nature and a 30-31IV has 80 Speed, so if it moves first and survives Night Shade, I switch to Smeargle, which I was fortunate enough to have on hand. The Smeargle is level 79 and has 4EVs in Speed, so it has 162 base Speed; with a Power item/Macho Brace, that's an 81 in Speed, so if Latios outspeeds Smeargle as Smeargle uses Spore, the synch has failed.
I'm going to do a Modest Suicune next, will be using this:
1. Fainted Synch in front
2. Staraptor (lv. 54)
-Spe Nature
Ability: Intimidate
IVs: 31 HP / 0 Spe
EVs: 12 HP / 24 Spe
- Final Gambit
Modest Suicune has 175 HP and 105 Speed if a 31IV. This Staraptor has 174 HP and 104 Speed. Use Speed O-Power, if Intimidate activates before Pressure, soft-reset. If not, Speed O-Power activates and I can safely use Final Gambit.
3. Smeargle (lv. 100) @ Macho Brace/Power Item
+Spe Nature
EVs: 28 Spe
- Spore
- Recover/whatever
This Smeargle has 212 Speed, which a Macho Brace that is 106, outspeeding Modest Suicune with one point. Use Spore, if Suicune moves first, synch failed and soft-reset.
A better set than this Smeargle might be used, idk though.
pls correct any mistakes I might have made I derped so much today I might have derped this too
Morning edit: Oh yea
vzxt is right, it doesn't guarantee your synch worked, it's just a much higher probability that it did.