maybe a good tester would be for you to 'organise' an event, such as a trip somewhere, or a smallish party at your house...
and see if they are firstly keen, and secondly whether they actually turn up to said event.
then if they are not keen, or just dont turn up you can guess the freindships over, and you should try and move on and find new freinds..
but on the other hand you could find that they are all really enthusiastic and pleased that youve arranged something.
a simple 'event' like inviting them to watch a DVD or something would be ideal.
also another thing id like to add, is that appreciating that freinds have other freinds is a hard skill to master (im bad for it myself) and that its easy to feel jelous when freinds are having fun with another circle of freinds, because you want 'your' freinds to yourself...im really bad for this! in some ways id even say (selfishly) that i dont want my freinds to have other freinds...but then when i think logically, obviously a person with only 1 freind would be a very dull person!
not saying you are doing what ive just written (to be honest its your freinds who seem to be in the wrong) but i think in general people find it hard to appreciate that other people have other freinds to be with.