How to identify data gaps in grid format since i have huge amount of data to handle

Nats

Let's say I've got elevation data of a country (500 km x 500 km)  in grid format and how to identify the gaps.

a) no data/data gap  or b) wrong data,

 Since this pixels are so small compared to the whole country, I'm not able to recognize them at first sight within GM. There just covered with the correct altitude pixels around them. Do you have an idea how to make them visible at first sight,  Right now I've to zoom in to be able to see individual elevation pixels and then pan the map acrosse the whole country, which is extremly cumbersome and time consuming

Need suggestions.