BBC Radio have been in touch.

They had seen last week's "Paul Says" about fake autographs and want us to discuss autograph collecting live on air next week.

And it's not just how to spot fakes we'll be talking about. We'll also be valuing callers' own prized celebrity autographs.

Which, over the medium of a telephone line, will be a little tricky!

But it set me thinking.

I'm sure the presenter will ask what makes for a valuable autograph.

Here's what we'll tell him:

What makes an autograph valuable?

Desirability: Will the signer be famous 50 years from now? Few names through history remain famous in the decades and centuries after their death. These autographs are the ones that have the most suitors today, pushing prices higher - especially if their signature is rare. Those who pass the 50-year test include icons such as George Washington, Marilyn Monroe and John Lennon.

Rarity: An autograph lots of collectors want to own but which is very hard to acquire is the perfect recipe for a valuable signature. James Dean, Amy Winehouse and Shakespeare all fall into this category. A word on Shakespeare. There are six known Shakespeare signatures in the world - all in institutions. If ever one should appear for auction, it will make millions. Good luck getting your hands on one.

Content: Signed photos are generally valued more highly than simple autographs. That's because they make for better display pieces. If buying a signed photo, try to ensure the signer is as he or she is best known. For example, Charlie Chaplin signed photos depicting him in his famous "tramp" guise are more popular with buyers than a signed photo showing him in civilian attire. Collectors love to see a signature that is contemporary with the photo, too.

Rare and coveted - James Dean autographs like this example in stock are valuable

Signed letters, if the content is important or illuminating, can sell for substantially more than signed photos. For example, a collection of five Einstein handwritten letters on the theory of relativity sold for $161,000 in 2013. His signed photos make around $15,000. 

Date: Timing is crucial when it comes to the value of an autograph. Winston Churchill signatures from his most famous days - the second world war from 1939-1945 - are his most valuable. The first and last known autographs are also much in demand, the latter especially so when the person died unexpectedly. The last autograph John F Kennedy gave - on the morning of his assassination - sold for $39,000 in 2010. In contrast, museum-grade JFK handwritten signed letters are valued at around $13,000.

The signature itself: Look for a clear and crisp signature, with no smudging or fading.

Condition: The better the condition of the item, the greater the number of collectors who will want to own it. Hunt for a signed piece with no creases, as well as sharp corners and edges.

Authenticity: The most important point of all. Is the autograph genuine? Will you be able to show prospective buyers it's genuine? If you're not absolutely certain, enlist the help of an expert.

I have 40 years' experience in the autograph business. I'm here to help.

Call me on +44 (0)117 933 9500 or email info@paulfrasercollectibles.com.

Thanks for reading,

Paul