1990 Subaru Legacy over heating. No fluid visibly seen but radiator is empty

Does the exhaust look smoky and smell sweet like anti-freeze?
Check your engine oil level and see if it is high or if it has a milky substance in it.
You may have air in the system as another possibility if it is not overheating and the above are not true.
I you are adding a pint or two and have done so once or twice, I would not be concerned yet. If you add quarts and have done so every day for a week, there is somewhere that the anti-freeze is leaking.
Have you had it serviced lately?
Let me know the above and I will get back to you ASAP.