I sponged all mine and then lambed them all in 3.5days made planning sooooo much easier, and well worth the £2.50/ewe it costs to sponge them. Other than that, camera's are a good option but if you are far away to work you would want to leave to get back to them as soon as you noticed the ewe was thinking about lambing as if you wait till there's a problem it will be too late.
Me and my wife both work full time but she took time off for 2 days (started on Thursday early in the morning, so took thurs & friday off) and she called me if there was anything complicated, only lost 1 lamb out of 48 due to it coming both legs back and it being huge and i didn't get back in time, but she was a triplet anyway so, it just meant one less to bottle feed.
C B