you can test the actual sensor itself with a gas blowtorch (if it's a Zirconia type sensor) and a multimeter

http://www.fpuk.co.uk/technical/ttl2.htm

BUT, please check with Stu first to see if the EsCos standard one is Titania construction, as I don't think it works the same way with the blowtorch test..

http://www.fpuk.co.uk/technical/ttl1.htm