The only way to tell for sure, besides shooting them and neglecting to clean your rifle, is to pull a bullet, dump the powder, and discharge the primer onto a piece of bare steel. Check it over the next few days for signs of rust. I would assume ALL surplus ammunition is corrosively primed, unless it is US surplus produced from the 1960s and later.
the only thing that MAY be corrosive would be the primer
Um, yeah, the residue from the primer is what is corrosive.
if a lot of mercury is used.
Mercuric primers are not corrosive, and have not been in general use for about 80 to 90 years. Mercuric primers damage the brass cartridge cases by blowing bits of free mercury into/onto the brass, which forms an amalgam with the mercury and weakens the brass.
Corrosive primers leave a deposit of potasium chlorate or potasium chloride when fired. Both are metalic salts. The salt residues are hygroscopic...that is, they absorb moisture from the air. The moisture sitting on the barrel steel is why rust begins after firing corrosively primed ammunition.
The best solution is to clean your gun appropriately after firing corrosive ammunition. The bore can be flushed with hot water, swabbed with surplusGI bore cleaner, or cleaned with an ammonia/water mix. Do one of those things first, then clean with your usual solvents, and it will not matter if the ammunition was corrosive.