Yup, there is no definitive answer for that. If the theater sets aside tickets specifically for rush than yes, they will be available even if the show is sold out. But that also likely means the show is very popular and rush lines will probably be quite long/you have to show up early.
Some shows however only sell whatever they have left day off, so if they are sold out that means no rush.
You could also consider Standing Room if you're desperate. Many shows have a SRO policy that they start selling once the show is completely sold out.
_IrisTInkerbell said: "Many shows have a SRO policy that they start selling once the show is completely sold out."
Same day only, though. If the show is sold out in advance, it is when the box office opens that day. Otherwise, if there are a few seats left, you have to wait until they are sold before most theaters will sell SRO.