I want to know when I have collected all the trading cards for a game. Is there a way to do this?
In Steam, simply click on your name on the top bar and then click on Badges - badges you have completed will be shown at the top of the list with a "Ready" button next to it. If you have not yet completed the requirements for the badge (ie: only have X out of Y cards), this will be stated next to the badge instead of showing the "Ready" button as follows;
This is what a completed badge with all collected cards looks like, before crafting the badge (and thus using up the cards), complete with a "Ready" button;
For a more detailed breakdown of the cards you're missing to create the badge, simply click on the game in question (for example, Left 4 Dead 2 in my screenshot above) and this will take you to a screen showing all of the cards required for a badge along with the ones you're missing, and information detailing things like which of your friends have certain cards, along with links to the Steam marketplace for quick acquisition.
Go to your badges page (your name > badges), click on the game you want to see the cards of and it will display them to you. You can easily see which cards you have and which ones you are missing. Steam will also display a list of friends who own each card that you're missing.
The page you're looking for should look something like this, amount of cards and the actual cards depending on the game:
(Apologies for localized screenshot)
Additionally you can see the total progress easily from the main badge page: