How to check how much of given asset can be borrowed?

For Aave flashloan entire available liquidity of pool is available to borrow. In case of stable or variable mode borrow, you can borrow max that can be supported by total collateral i.e. max amount with HF >= 1

available liquidity = totalSupplied - totalBorrowed

in case an asset has debt ceiling

available liquidity =min( totalBorrowed - debtceiling, totalSupplied - totalBorrowed)

More info.