I have found film reels some with tails of film others not. Is there a way of telling if they have been exposed/used in a camera? Film types are for 35mm and one Kodak Advantix.
As for the Advantix (APS) film, check for a white circle on one side of the roll.
If only a half circle or an X appear in white, there'll likely be some pictures already taken: in the former case you'll be able to insert it into a camera supporting this feature and have it automatically start after previously exposed frames.
While this technique isn't foolproof (resetting this indicator was in fact a requirement to replace the film at any time with cameras not supporting the resuming of a roll) it should be a significant tipoff; with standard 135 film there is no such option and position of the film as it leaves the camera tells nothing about its contents, with no practical way of checking.
If you hold any expectations on their contents, have them developed at your local place (empty rolls are generally charged at a relatively low nominal price, since there'll obviously be nothing to print).
The only sure way to tell if film has been exposed or not is to develop it. Any light used to examine undeveloped film will fog it.
It is probably safe to assume the rolls with no leaders extended out of the cartridge have been exposed. The rolls that still have film showing are another matter. Some cameras with motorized winders, such as my EOS film camera, would wind all of the film back into the cartridge after the roll had been shot. Some other cameras with motorized winders had an option in the settings that allowed you to leave the leader out. With manual film rewinders, the decision was up to the operator. Most folks would wind all the film back into the cartridge, but some would watch the exposure number indicator and stop when it got down to "0".
If you know what camera the film might have been exposed in, take a look at the takeup spool in the camera. Many cameras have a winding mechanism that involves some kind of slot in the spool into which you stick the film leader, and this bends the leader as film winds onto the spool. If the exposed leaders have a discernable bend or crease, that's a telltale sign that the film was in a camera at some point. If the leader is smooth you really can't say for sure -- IIRC cameras with integrated motor drives grabbed the film a different way and may not bend the film.
So I just found this thread and discovered a way to solve the problem actually. My Nikon L35af (I think...) leaves a little bit of the roll out when it rewinds. This is awesome when you want to shoot double exposures but a bummer if you forget whether or not you shot the roll yet. I compared the number of sprocket holes to a brand new roll of the same film - exactly the same. I looked at the end of the lead but there was no bend.
Finally - I noticed that the roll had very tiny scratches on it in the glare from my bedroom light, I compared the glare to a brand new roll and it was completely smooth. When in doubt try this method and see if it works for you. :)
Unused Kodak 35mm rolls have the tail, plus the ramp, right up to the full width of the film, exposed for three sprocket holes.
In my experience, auto rewind cameras set to "leave tail" will either rewind until all of the upper row of sprockets are within the canister, or with a longer length of the full width of the film than three sprocket holes. (IIRC, it was an option to choose between tail-out styles on some cameras, as the "more lightproof with film in slit vs. more lightproof with tail fully rewound" was a favorite flame war of the primordial pixel-peepers back in the day.)
You may have run into a roll of film where the photographer fiddled with it until there were exactly three sprockets exposed, or the film may have expanded or contracted on the spool over the years so there's two or four sprocket holes exposed, but it's a good guideline.
A GIS for "Fuji 35mm film roll" indicates that either 2 (Fuji Pan) or 4 (Velvia) sprocket holes are exposed for that company's product. I think it's a good bet that anything less than 5 or greater than 0 means it's unexposed
I use Fujifilm and Ilford, and I find that they have the tab, ramp and about 2 sprocket holes of the full width film outside of the casing, as new. This is a good indication that the film is 'new'. It is a million to one that you rewound a film just exactly to the point that the manufacturer sets! Once, I had a film that I was sure I had not used, albeit, it only had the tab exposed (strange: I know I have not used it, but it doesn't look like a new roll): Carefully, with my fingertips, I pulled out some of the film so that the ramp and a couple of sprocket holes of the full-width film were exposed. Had a look and saw fingerprints that would not have occurred in the factory. So it had been exposed after all!
Better photo labs will offer a service called a "clip test". This is where they extract just the first few inches of the film from the cassette and cut (clip) off that portion. They'll develop just that portion, and in an ideal world, it will give you an indication of how to proceed. Note that this can also destroy a frame, because they might cut right through the frame.
If you are getting a lab to do a clip test in order to determine if the film has been exposed or not, do tell them – it could be useful to know the purpose of the clip test in advance.