The best way to find out which ERC20 assets we can use within AAVE flashloan?

Нou can use getReserveData on Aave Pool to see if borrowing is enabled for that reserve. If borrowing bit is true then you use Aave flashloan entire available liquidity of that reserve.

availableLiquidity = totalSupplied - totalBorrowed, which you can compute using info from AaveProtocolDataProvider getATokenTotalSupply and getTotalDebt.