There are a couple of different ways of finding out the value of shares: namely looking in the financial pages of daily newspapers (like the Financial Times) or online using websites such as Yahoo! Finance or MSN Money. Once you know the price, you can do some further research into the prospects for the company and decide if you want to sell or hold on to them.
When it comes to the Leeds Building Society shares, if these were the windfall shares after the merger with the Halifax and you haven't actually claimed them, I think they are now lost to you – unclaimed shares were sold in 2001 and while there was a nine-year window to reclaim the proceeds, that has now passed. If you did claim and have share certificates, you could go to a stockbroker for advice.
If you have lost the share certificates, you can try to locate them by contacting the registrar that issued them.