How do you scout battle cruiser openings in TvT?
Fusion Core is 99% sign of BCs.
Another sign of BC’s is if you scout your opponent around 5 Minutes and they barely have any ground units, and you don’t have a medivac dropped in your main from the enemy.
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An early fusion core (very soon after starport completes) is almost certainly a BC opener. Mid game, multiple tech-labbed starports is also a giveaway.
An early BC jump in is easy to deal with. You should already be building a cyclone as your first tech-labbed factory unit, as it helps deal not only with BCs, but also liberators, banshees, hellions and reapers, and you should be producing vikings fairly early off your starport under all TvT scenarios, as air control extends the range of tanks and is very important in TvT vs tank pushes. A cyclone and a viking, along with a few marines, can easily deal with a BC. You just need get a cyclone lock on © and then micro it out of range of the BC.
Players in lower leagues will usually 1 base all in BC rush. To scout for this, use a standard reaper opener and check for:
- A lack of tech.
- Lack of expansions
- Lack of units
- Double gas early
Of course you can also use an scv to scout for the double gas using 17 scout. 
In higher leagues, you will have marines or some type of scouting unit around the map to check for army movement. This can also help with scouting proxies I hope I helped you 
Edit: I forgot to mention that if you see a techlab and something researching on a starport, it could be a fake but its most likely a banshee harass. Else, it could be a raven or BC.