How to determine the direction of regulation of a gene by comparing gene expressions?
If you have control expression values $c$ and e.g. disease expression values $d$, you take the ratio: $\frac{d}{c}$. If this is greater than one, it's up-regulated. Usually, the log-ratio is computed: $log\frac{d}{c}$. Now, if this is positive, the gene is up-regulated. 

Gene expression values are usually measured genome-wide and then normalized before computing the ratios. So you rarely deal with individual raw expression values.