i have just bought 88 silverado, an the gas gauge reads half tank all the time.....

is problem the gauge? or sender unit?

 

As far as the popular culprit I think the sender acts up more than the gauge, however it is not set in stone and could be the gauge as well. As far as how I would go about checking it - I would unbolt the bed (easier than it sounds) and pull the pump and sender out to see if it is stuck. Unless you have access to a similar truck that you can swap entire clusters to see if that helps. The cluster swap is definitely easier, but from my experience the sender is usually the issue.