What is the name of the variabel you used? Then add ‘ != empty’ behind it.
Let me know if it works… Or if you meaned something else.
In addition to Mark's response, instead of
$Variable != empty
You might want to use
length(trim($Variable)) != 0
We use the latter for checking whether strings are filled, because they can sometimes (e.g. after editing it to empty on a frontend page) contain an empty string. This method first trims the $Variable, taking away all whitespaces before and after any text in the string, and then checks whether the resulting length is not 0.
Pick which one you want, the bottom one also takes care of empty strings :)
Note that this only works with string values. Booleans and integers and such you should check with != empty.