If it's an old bike, it will almost certainly have a 1" steerer and a conventional threaded headset.
If you have the old forks, it is easier. Measure the diameter of the "steerer" to make sure. Also measure the length of the steerer and try to find forks which have a "rake" similar to the old ones.
If you don't have the old forks, the two things you need to know are what wheel size (26", 27", 700c) and the length of steerer you need. If you measure the length of the headtube (through which the steerer fits) and you then need to add a bit extra to allow for the headset races, locknut, spacers and possibly a headlamp bracket depending on the bike. It is obviously easier to cut a bit off than to put it on so my advice is to buy a fork with a steerer which is about 1/2" longer than what you think you need. They usually have a lot of threads. You can always cut the excess off with a hacksaw (thread on an old bearing race first, to re-shape the threads after the cut). The other thing you might need to be aware of is that old Raleigh forks take a non-standard headset.
