[Ethereum] how to know the contract address which truffle is deploying with

By default, truffle uses the first account but in truffle.js you can specify the account address in the from property:

module.exports = {
  networks: {
    development: {
      host: 'localhost',
      port: 8545,
      network_id: '*', 
      from: '0x0c439ff0170Ab7d7Ef55f2993554eede7321c9b7'
    }
  }
}

If you want to deploy to a network other than localhost then you need to import a wallet provider and pass it your mnemonic.

Here's an example where the mnemonic is read from an environment variable and the provider is set to rinkeby testnet:

const HDWalletProvider = require('truffle-hdwallet-provider')

const mnemonic = process.env.MNEMONIC

module.exports = {
  networks: {
    development: {
      host: "localhost",
      port: 8545,
      network_id: "*" 
    },
    rinkeby: {
      provider: new HDWalletProvider(mnemonic, 'https://rinkeby.infura.io'),
      network_id: '*',
      gas: 4500000,
      gasPrice: 25000000000
    }
  }
}

This answer might help https://ethereum.stackexchange.com/a/34374/20276

Along with this snippet:

const Web3 = require('web3')
const web3 = new Web3('https://mainnet.infura.io')

function myFunction(myParam) {
    var contractAddress = '<ENTER-YOUR-CONTRACT-ADDRESS-HERE>'
    var contractABI = [<ENTER-YOUR-ABI-HERE>]
    var contract = new web3.eth.Contract(contractABI, contractAddress)
    var callData = contract.methods.<CONTRACT-FUNCTION>(myParam).call()
    callData.then(function(result) {
        console.log(result)
    })
}

myFunction(<MY-PARAM>)