detecting overflow in arithmetic left shifter

Hi, I'm trying to implement an arithmetic left shifter in vhdl. I used barrel shifter for a 16 bit number (4 levels of 2-1 muxes) and I'm wondering how I can detect overflow in shifting. I know if I shifted out anything that's different from the sign bit I'll have an overflow, but how can I efficiently detect that (using xors?) What would be the least amount of logic to do it?

Thanks in advance.

dlamoris