Assalamo Alaikum Wr Wb!
Dear @ahmed,
Three main conditions have to be met for a permissible animals slaughter to be classified as halal: 

1. The animal/bird must be alive at the time of slaughter
2. All flowing blood must be drained out
3. A Muslim should do the rendition of Shahadah/Tasmiyah at the time of slaughter.

Althought there are many other desireables which would be preferred, but if these conditions are met, then the slaughter would be classed as halal!
As far as the use of machinery/machines for the purpose of slaughter is concerned namely machine slaughter, there are various opinions among the scholars, but the majority view point among the Fuqa’ha (Expert of Fiqh) is that as long as the above conditions are met, it would be classed as halal.
Now the problem lies with the monitoring and the actual use in modern slaughter houses. Most halal monitoring/certifying organisations in the west don’t have the powers to implement procedures and continuous monitoring of the slaughter procedure, which has created confusion and has led to distrust among consumers.
So the best advice would be “if in doubt, leave it out!”
And only Allah knows best!
Ma’assalam,
Some references:
http://islamqa.info/en/75891

http://www.isnahalal.ca/doc/Dr.Siddiqi.pdf
http://hma.jucanada.org/mechanical_slaughter.aspx/user_submission_system/machine-slaughtered_meat