In reply to alpchild004: Sadly, the only way is look the night before ( and see what the weather conditions have been in the prevailing days)....there may be wall to wall snow from Llanberis pass upwards, and you can climb anywhere; or it could be wet/shitty and you need to go canoeing.
Welsh winters are not reliable for forward prediction ( if indeed any winter venue is in the uk) - the classic welsh winter climbs p9 has a great section on this - you need to be able to drop everything and go when conditions are in ( as they can go overnight!)
in general cold sustained weather for a week will bring in low level water ice routes; but higher crags could well be poor, and the gullies limited and powdery.
on the other hand a succession westerly freeze/thaw cycles will lead to snow build up high up, and subsequent consolidation and will bring higher routes into nick eg Snowdon trinity face. whilst the ice routes eg the kitchen, will be struggling to form any thickness, and the ice platey if there.
one of the things I've noticed on this site in the recent milder years is a view that as soon as its cold or snows, winter routes are in. Apart from top grade modern technical snowed up rock routes, which can come in very quickly, most routes need a few days of frost to build up and/or a few days for the snow to consolidate. Without getting technical ice and snow, change tremendously in character as they metamorphise. Alot of routes require these changes to occur to get the best ( if not indeed acceptable) conditions to climb upon.
a great 'mountaineering' day out can be had in all conditions though, best of luck