First of all since you have the variable speed Carrier unit there is a place on initial start up that is for charge calculation. The installer enters the line length and size and the system will tell them how much more or less is required to charge the system. It also lists a target superheat setting that can be checked in the cooling mode. If they had to add ANY refrigerant after the initial start up then they either did not follow the proper procedure as described in the installation instructions or it has a leak...PERIOD. If they tell you otherwise they are full of sh--.
(Is there a way to check or monitor for Low Refrigerant using my carrier infinity control (SYSTXCCUIZ01) so I can alert the service tech and make sure my heat pump is running optimally? Or is there some other way for me to check?)
There is an old school SWAG trick you can try. This is for 10SEER R-22 systems but is still scary close. It is called the "+60" method. You first need to know the Outdoor temp. let's say it's 60* and you add "+60" to it and you get 120* coming out the closest supply air to the A/H. With this said at 40* "+60" you will get 100*, at 20* "+60" you will get 80* out the closest supply grill. While 80* feels like cool air it is still heating the home. Again this is not in stone just an old trick that I still use and hasn't let me down. SWAG stands for "Scientific Wild Ass Guess". I wouldn't use this to call your "service company" becaused it depends on wheather or not or how much your coil is frosted ,condition of filter ect. but it will give you a rough idea of how the system is working.