There really isnt any way to tell for sure until 30 minutes before departure when check in for that flight is closed.

Jetnet is a good guide for non revs, it is not an accurate tool for flight loads and seeing how sold a flight is .One example is that it shows you how booked the arcraft is, but not how many passengers are ticketed. It will not show you duplicate bookeings etc. It might say it is overbooked by 5, but three of those passengers might be on a 30 minute time limit.

To really see what is going on then you need to be able to log in to Sabre . To be honest, if your Mom is not really computer savvy then this is probably not an option for you.

Good luck though.