You can use a typical thermometer used for determing indoor/outdoor temps and place it in the freezer. It should read 0 to 10 degrees F (I'm in the U.S.). After determing that fact, if your fridge/freezer is a frost free or automatic defrosting type, you could have some issues with either the door seal or, more probably, the defrost function.
If it's the defroster, you'll need to completely thaw out the freezer (removing and storing everything, since this is going to take a while), and at the back of the freezer section you should notice some small screws that hold the plastic liner in place. For the average DIY'er it's not difficult to remove this panel. Behind it will be one or two devices that look like small, cylindrical light bulbs perhaps 5 to 7 inches long and an inch in diameter. These produce the heat to defrost the freezer during its cycle (the melted ice then runs out the bottom through a drain hole into a tray under the unit). One or both may have burned out. They're easily and inexpensively replaceable. If this seems a daunting task, call a service person...
It may just be a door seal that's letting moist summer air in that's freezing up. Again, best plan in any case is to defrost the whole thing and get it cleaned up...
Best of luck!