Most auctioneers are happy to give verbal valuations for free without an appointment. (They charge if you want a written valuation for, say, insurance purposes though). That's because they're hoping that you'll sell your items through them.
I've taken stuff into Bonham's in London (which ranks alongside Christie's and Sotheby's in the prestige stakes) and simply spoken to someone on the reception desk about the items I'd got. A few minutes later their expert was giving me a detailed appraisal of the things I'd brought. (He was the same guy that people queue for hours to see on the Antiques Roadshow, so he ought to know his stuff!). All for free!
Be warned though that many items which people think might be valuable only raise a few pounds at auction. (Attending local auctions is a bit of a hobby of mine, even when I'm neither buying nor selling. I can usually estimate quite well what certain items will sell for).