Just as a quick point of reference after a few questions on it; what I'm looking for in readers varies depending on what point in the process we are. For an ARC, just a review on any site (Amazon, Kobo, Goodreads, etc.) you can fling it is awesome - same way you would any other book you read.
If the book is in the beta stages, I'll sometimes send out some questions I'm particularly looking at having answered. I'll put the questions at the end of the eBook because I know some people like to do a clean read with no outside influence (but, there will be a link to jump forwards and back in case you want to read them first). And, they'll also go up in the relevant forum category for you to peruse at your leisure.
If there aren't any specific questions, it's pretty relaxed:- what you liked, what you didn't- what you thought of the characters (good and bad things)- what you think could be removed, or something that could be added or you'd like to see happen
- any slang/words/conversations you didn't really understand or want explained (I'm hearing there's a bit of this from non-Aussies and I'd love to know more about which bits they are)- and just any other general thoughts you have on it.
Most of the time, a beta read is mostly like an extended review - but one in time for me to make changes before it goes off for publication. I'd love to get an idea of how you feel while you're reading it. What are the slow parts, the fun parts, the emotional parts? It helps me to work out if I'm succeeding with my intentions - getting you to smile when I mean to, rather than just totally by accident. A Beta Read response can be whatever you like really - from a couple of paragraphs to a few pages - no amount of feedback is too much or too little, everything is helpful.
Remember, if you have questions, I'm pretty much always on email, or you can post them here or on the relevant forum category.