How do I know if my rebatched jelly has set?

You can tell your jelly has reached the setting stage when a little is placed on a cold spoon and the drops run together and "sheet" off the spoon. Another option is to place a small amount on a cold plate and put it in the freezer for a few minutes. If it gels it should be done. Cooking the jelly to 220°F is another way that works well for jellies without added pectin.

Can you recook jelly more than once?

Yes, if you need to, you can recook your jelly multiple times. But if you keep having failed jelly consider trying one of the other methods to fix it.