Using TextMate on Mac, the "invisibles" can be displayed by using
View -> Show Invisibles
But the space vs tab characters are not shown, and Preferences doesn't seem to have any entry to change that either. Is there a way to show them? It is because some older coder may have tab or space mixed together so what looks good in the editor may actually be misaligned. (editor shows tab as 2 spaces)
If "Show invisibles" is activated, you will see a little triangle for each tab, which points to the right. Spaces are not shown at all, you can only see them indirectly, i.e. via the symbol which represents the line break. If the line break isn't the first character after your code of that line, there are spaces in between.
So to make a long answer short: You should already see tabs, however the symbols representing the invisibles are sometimes hard to see.