I just had to go through this when making size 14 socks for my father, and with fine yarn, 1x1 ribbing and mailing them a distance, wanted to make sure they fit!
There are some very general rules I know from old fashion design and fitting rules...
This one is APPROXIMATE but helpful if you are unable to measure the foot of the recipient:
If you know the person's height in inches, you divide it by 6 for a man, and 7 for a woman.
This one is exact, and can throw off many as to what the gift is to be:
Measure the bottom of the forearm, from the outside of the elbow to just beyond the wrist bone. (A bend arm makes this easy and accurate). This will provide the precise foot measurement.
There are also charts online you can search. If you know the person's shoe size you can measure the inside of that, but vendors vary and men tend to wear their shoes looser, and women sometimes on the tight side so it isn't a completely reliable indicator, but it will provide an approximation.
Obviously #2 is the most reliable and with all of the work that goes into socks, the one I rely on if I can't measure the foot itself.
Hope this helps!