Is the back yard lower than the front? If so, that would mean the water was blown out of the lower elevation heads and not to worry. These guys should be liable for any pipes that are damaged during the winter by water left in the pipes. Most companies know this, charge appropriately and make dang sure the job is done correctly. It isn't a tough thing to do and most companies always include the recharge of water in the spring.
If you don't have this service with this company I would call them back to request they come out in the spring to recharge. If anything is broken, they are responsible and they fix everything at that time as well. Find out now. If you can get the same company to commit to the spring recharge and they've been around for awhile you can probably rest assured. If they want to be paid AGAIN for the spring recharge, find another company that will assume responsibility and that will blow your pipes (or check on their status) AND recharge in the spring. You'd have to pay for the recharge anyway but at least you are getting an 'insurance' policy for your irrigation. The guys that blow them out need to retain the responsibility. I've never known a company not to be responsible for the spring charge if they've done the blow out...all included in the same price. Not separate prices!
Watching them doesn't help much. If you know what you are looking at you could have done the job yourself. Making sure that the same company is under contract for blowing out your pipes and recharging AND taking responsibility for any winter damage is your best assurance!