I've seen a lot of these signs affixed to houses which just show alandline and mobile number, but no other contact details.

I know that things are done differently but I've seen next to no estate agent signs as we would recognise them in the U.K.

Are all these plain Se Vende signs private sales or, if agents as well, how do we tell the difference?

Normally, they are from private people. Sometimes, they are from "professionnals", but no legal at all... A proper real estate or agent's sign is easy to identfy, because at least their name appear on the sign.

Hola

But you don´t get the proliferation of signs you get in the UK

Davexf

The most annoying thing about professional signs (estate agents) is that they are glued to the side of buildings.

When they are removed, the signs of glue remain - yuk!!

anyone can buy a Se Vende sign in the local ferrateria, they use the same ones on houses and cars.

if the contact number is a mobile number and it isn't the same number plastered around town then it will probably be a private sale
professional Spanish estate agents like to have their business name shown (it attracts more sales business)

otherwise just ring the number and see what sort of response you get

good luck

My own experience only..I have a se vende sign outside my house with the relevant phone numbers on, no estate agent sign. None of the estate agent's that are advertising my house has affixed one!!
I also put a plastic envelope with a4 sheets in, usually ten at a time, with all the details of the house,in English and Spanish,with links to the agent's sites on too. This was affixed to the door-way so people wandering by could take a sheet and as there were photos on it,all the info they need was there. Unfortunately, I had to take the sheets in because one old man used to take them for toilet paper and I got fed up with him waiting around for the envelope to be re-filled. They may have done some good(especially for the old laddo) but then again.....