Q) How can I tell if my mash is ready to run?
A) The best way to tell is by taking a starting gravity reading with a hydrometer and then taking a final gravity reading to see if it done. This article explains it pretty well: https://www.clawhammersupply.com/blogs/moonshine-still-blog/11938081-making-moonshine-how-to-know-when-fermentation-is-finished-part-2
Q) and how can I tell if my mash has gone bad?
A) As long as you use good cleaning and sanitization methods you should have no issues- Also I recommend fermenting the mash in a food grade fermenter with an airlock. If you see anything “funky” of if the smell is terrible those are two easy ways to spot a bad mash.
Q) How long can I keep the mash before I run it?
A) As long as you have an airlock on your fermenter you can keep it for a while. If you plan on leaving the mash for over a month I would recommend racking the mash to a secondary fermenter to get it off the yeast cake.