Generally it's quite obvious. You can tell if the plastic was originally that shape or re-shaped to fit.
Edit: I've decided to post this here.
If you like modifications, you should take a look at these.
http://www.youtube.com/watch?v=y42vBw1hp_A
This was how local tournaments used to go. Perfect circle form beyblade with tuned and seasoned bearing core. It's constructed from
AR: Bound Defencer cut to accomodate support parts, with Cross Survivor (dranzer v2 support parts) filed smooth
WD: Wide Defence, filed to remove the bump
SG: Bearing core 2, two tuned bearings, with either Wolborg2 shaft or Wolborg shaft. The tip is cut flat for wolborg2 shaft.
BB: Dranzer S, cut to accomodate bearing core 2 and support parts
SP: Another set of cross survivor, filed smooth.
The perfect circle form gives perfect zombie type, except that the core is using a grip core (this is meant to be general purpose). It can be beaten with another similar one, but with a smoother tip and better bearings and such. However, as a general purpose tool, the grip version effectively removed ALL competition from the tournaments. There was
no way to win tournements without using this or similar. The highest I've heard of was someone making his way to quarter-finals by carefully picking his opponents. Beyond that, all match-ups are supervised by the tournement organizers.
When I went for tournaments during this era, the older players would launch this into the center of the stadium to make sure the flat tip doesn't try to attack. Knock-out at the start of the battle is the only effective way to win this, and experienced players would grab the stadium center as soon as possible to prevent this. I stopped going after two tournaments because, essentially, there was no point going unless you intend to use something like this. You can easily see the difference in survival capability between these mods and an official beyblade.
I believe this video wasn't made by the older players. The usage style of this is totally different. You can see the players launching at the sides instead of the center. But you can still see the difference in the 3-way battle. I think the first battle is using a smooth core version rather than the usual grip version.
Thankfully, eventually, they banned this just before HMS era. Unfortunately, due to waning popularity, that basically meant that by the time it was banned, there were only a handful of tournements before the importer decided to stop holding official tournements anymore.
It's because of things like this that I cannot agree with modifications.
Besides this, I'm sure you've seen what happened on Japan's side. Locally, the rule was "as long as it used official parts it is allowed". But it doesn't say that you cannot modify the official parts to make it work in the way you want it to be. Initially, in Japan, the rule was that you could even CREATE your own parts. Soon, because of parents custom-making Beyblade for their kids, tournaments actually became dangerous for health. The term Bey
blade actually had meaning.
If you want to discuss illegal modification, you shouldn't do so in the "Beyblade" section. Illegal modifications are a completely different game. The fun of beyblade comes from working within the rules. It was designed to be as balanced as possible- to prevent dominance of zombies, in the HMS era, better attack was introduced, and so on. I expect MFB to be better designed, too, to prevent this kind of monopoly.
Anyway, just in case you're wondering, "Team Armstrong" was set up locally to dominate tournaments. Locally, Beyblade has always been a game of "who has better parts". After the mod ban was removed, near the start of HMS era, I managed to do well because of importing WybernDJ and having good parts like the Metal Sharp core (as compared to flat, semiflat and sharp. Only one single player there, other than me had DragoonMS's grip flat). At this time, the playing field was still quite even because there were only five HMS beyblades released. Later in the season, they invested to import many Samurai Changer, RoundShell, Jiraiya, etc. long before these arrived locally, and continued to dominate tournaments again. Tournements stopped just after that, and it took a further few months for these beyblades to be imported. Up to now I still don't know the reason for the delay, although many people suspect foul play.