Get a ballpark value for it, consider what you would pay for it if it was broken, and then make her an offer for it at that price. If she doesn't like the price you offer, you tell her without testing it out, you have to assume it will need repairing, and you thought the price you were offering her was a good one for a non working amp of that type.
This is how I bought my three McIntosh pre-amps. The seller didn't have an amp or speakers when I went by because he was getting the 3 systems he had inherited in bits and pieces, so that was how we negotiated. I took a chance and it paid off. The seller actually ended up selling me a few more pieces, mind you without the broken unit discount. In fact, he has given me first dibs if he ever decides to sell his one remaining MC2505. So it can be well worth it to take this tact for both parties.