How to know if your Steam account is premium?
According to this Steam Support page, the features that aren't available to limited users are:
- Sending friend invites
- Initiating chat sessions
- Voting on games in Greenlight
If you know anybody on Steam, the first two pretty easy to check. Either try to add him as a friend, if he hasn't already added you, or try to chat with him, if he has.
The third way is probably easiest though. All you have to do is go to Steam Greenlight, find a game you would buy if it got on Steam, and then indicate that by voting.
But really, it's not that hard to notice if you have any funds in your Steam Wallet or non-F2P games in your library. :P
Starting from April 19, 2015, the requirements for a premium account as well as differences from limited (free) account has been changed. (Steam Limits Users Who Haven't Spent $5 to Prevent Spam, Phishing - GameSpot)
These changes are mostly intended for reducing spams, but it will also affect users who only buy retail versions or bundles outside of Steam.
New Premium account upgrade conditions:
- Buy games directly from the Steam store and spend at least $5 USD total (you now need $5 total to access Premium status, price didn't matter until this changes)
- Fund your Steam Wallet with at least $5 USD (this condition is unchanged)
You NO LONGER GET a premium account for:
- Activate a retail Steam game
- Redeem a Gift or Guest Pass
Premium-only features
In addition to Sending friend invites, initiating chat sessions, and voting on games in Greenlight, the following features are now Premium only:
- Voting on Steam Reviews and Workshop items
- Participating in the Steam Market
- Posting frequently in the Steam Discussions
- Gaining Steam Profile Levels (Locked to level 0) and Trading Cards
- Submitting content on the Steam Workshop
- Posting in an item's Steam Workshop Discussions
- Accessing the Steam Web API
- Hosting a CSGO server
You can do any one of the following things to get a premium account:
- Activate a Steam Game
- Buy a Steam Game
- Fund your Steam Wallet
- Redeem a Gift Code or Guest Pass
If you're looking to trade items, you'll need to enable Steam Guard and leave it on for 15 days before you can trade.