The belt should be visible - and you can tell if it is worn - i.e. are the edges frayed, are there any holes in the belt etc. I replaced my alternator belt a couple of weeks ago. It was badly frayed at the edges plus had a couple of holes in it - so it was pretty obvious it was knackered. That was at 106k - but had more to do with a couple of visits to a gravel trap at Donnington Park a few weekends ago.
I can only talk about the 2.0 8v, but I'm assuming that the setup on the 1.4 will be similar. The alternator will be bolted onto the front of the engine block, between it and the radiator. There should be two bolts - one at the top of the alternator, visible from above. This one will be the pivot point. There will then be another bolt underneath the alternator. That will go through an oval shaped hole in the alternator - i.e. when you loosen the bolt, the alternator should swing backwards down towards the engine block.
To get some more tension in the belt, you need to lever the alternator towards the radiator/ front of the car. You should see that it has moved a bit as when you do the bolt up on the oval hole, it will bolt down in a different place.
If you are concerned that the belt might be worn then you can get a new one from GSF for £7.50 from memory. To fit, you just loosen the alternator as described but remove the lower bolt, so that it rests back on the block so there is no tension on the belt. Then you can take the old belt off, fit the new one, and then lever the alternator out towards the radiator/front of car again to put the tension back in the belt, refit the bolt and do both of them up.
Hope that helps.