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Eugh, there's never a good reason for that scenario.

1. Ask real questions about why they don't have the source, who has the source and why they did not hand over a copy of the source.
2. If there's even a slim chance, try to get the source from whoever has it (or have the client make a real attempt). It might be uncomfortable but worst case is they say no and you're no worse off.
3. Still no dice - you're going to have to do some forensic coding.

Get started by prettifying the code, looking for libraries, and generally try to work out the general age/era of the code. Half of it might be libraries, which you can separate out to get to the actual bespoke stuff.

The CSS and HTML should be pretty readable since stock minification usually doesn't rewrite the names of class and ID attributes. JS is a messier situation, particularly if the methods have been minified away. But you should at least be able to find selectors and trace where they are used, which might give you some hints for the DOM stuff.

If you have piles of minified business logic, it's probably time to bail out. It's going to be too risky to do much in there.

And charge by the hour for all of it ;)