There is a very glossy (coated) look to hot bluing. As compared to a deeper look of rust bluing. A trained eye can see the difference. The way the barrels are polished is a big indicator too. If there are polishing lines against the direction of the tubes instead of with them, that is also an indication of hot bluing. I say this because anyone who does not know the proper bluing pricess for a SxS is likely also not familiar with the proper way to polish a SxS. They would polish them the same way they would a rifle barrel.
Rust weeping out of cracks and crevices are an indicator too.
Hot bluing may never effect the solder joints at all, or it may quickly. Every barrel set is different. It may not start to come apart until they get hot from high volume shooting or until rust starts to take hold.
