There are three broad classes of trains, when it comes to seat reservations. Electronic Displays, Card notices, and None!
Looking at Electronic Reservations, these are generally seen on Cross Country + Virgin West Coast Voyager trains, and Virgin West Coast Pendilino trains. With these, you'll see a seat somewhat like this:
Up above the seat, built into the luggage rack, you'll see a scrolling display which tells you about the seat reservation. If the seat is reserved, it'll say when, which could be multiple times on a long journey (eg Southampton to Reading + Oxford to Birmingham New Street). Some seats might be reserved en-route, it'll say so if that's the case. Others are unreserved. Displays look like this:
On most other long distance trains, such as First Great Western, Virgin East Coast, Cross Country HSTs and the like, card tickets are put into a special holder just above the seat. On that will be printed when the seat is reserved, such as this one:
If there's no ticket card thingy above the seat, then either it isn't reserved, or there has been a problem with the seat reservations printer... The latter is rare, but does happen, and is announced if so. If your seats look like this, they're free!
Finally, there are commuter and outer-commuter services, which don't support reservations. First class seats on those typically look like these:
If a seat is reserved, but not yet, you're welcome to sit in it until the station it's reserved from. If it is supposedly reserved now, but no-one is sat in it after departure, you can sit in it. However, be aware that the seat reservation holder might've got on in the wrong place, and may arrive a few minutes later to turf you out...
In standard class, 4 seats round a table are generally very rare on long distance services, so unreserved ones get sat in quickly. For standard class, reserve if you can!
In first class, they're more common, but still often the first ones taken. Outside of peak times you should normally be fine to find an unreserved one to sit in.
(These photos taken by me today. I got lots of very odd looks taking photos of empty first class seats over a few hour period on different trains, I hope they help!)