How would you know if Right issue that comes with discount is good to subscribe or not?
Try to know the intention of the management for the right issue. What they plan to do with the money they are going to earn by selling and diluting the equity shares. It can be for the correcting the troubled balance sheet. Right Issue are sold where Balance sheet is already very leveraged and it would be difficult for the company to raise money from other sources. But investors need to know exactly what company plan to do with the additional money.
Calculate "Ex-rights value per share". This will help in deciding whether the discount is really a discount or not.
Read here in details to understand Rights Issue of Equity Share.