How to check the external I2C devices if they are connected

I have a Arduino Nano and 5 external I2C devices connected with I2C on pin 4 & 5.

When my sketch is starting up, it not detect if a external I2C device is connected. (i have to do this check only once at startup).

i want to create something like

Wire.requestFrom()

and when a external device is not connected, it return with a message

 lcd.print("I2C Error Device 2");

I dont know if i am on the right way. some help are appriciate.

thanks

Look at the techniques here:- http://playground.arduino.cc/Main/I2cScanner