Any pile of coin that's available as an input to a transaction is "unstaked" - it's either too young or it's currently "staking" (trying to solve a block). The chance of those piles "staking" is based on the number of coins and the "age" (i.e. time, number of confirms etc.) which combine to give the "weight". In that coin control screen you can see how the coins are split up (number of inputs per address) and the number of confirmations. This gives you an insight into their weight. Every pile of coins can potentially stake, although it has been pointed out that if less than (or equal to?) 0.1 coins it effectively has 0 chance.
As Superclam says, when you send coins you might "break up" a partially used input and the "age" for that input (chance it will earn a reward) resets. This is not good and should be avoided, as it will mean that these coins will take that much longer to stake.
What's unique about CLAMs over traditional POS is that while logic might suggest that you should have all your coins in one pile (number of coins adds weight) that doesn't work out so well in practice because the reward is fixed(*). While your larger pile of coins has a better chance of solving a block than a smaller one (at the same age), a set of smaller blocks will all eventually stake anyway, potentially giving you a higher return (multiple rewards rather than just one). How you split up your coins to maximise returns is up to you to decide.
"Staked" coins don't appear as available as inputs in the coin control window - their total is displayed on the main screen as "Stake" and are unavailable to spend for 510 confirms.
(*) Although the standard reward is fixed (0.1 CLAM) there is a chance of a (much) higher payout should you be lucky and hit a lotto block (up to 1,000 CLAM). Each time you solve a block there's the potential to "win" one of these, so it's another reason why multiple smaller piles of staking coins is much better than a single larger one.
Hope the above helps!
OK thank you once again
Just so I've got it right...... I just leave wallet running with a positive balance......eventually next to Stake it will change from 0.00 clams to positive number clams, and should remain active until clams balance changes