On defense, there are attitude, suit preference, and count signals. My guess is that the play to the first trick would represent an "attitude" signal for a suit. Beyond that, how do I differentiate between this, and one of the two other signals. Is it mainly the context of the play? Are some signals more useful in no trump hands, and others in defending suit contracts?
It's a good question - in Bridge we spend a lot of time memorising the precise significance of all sorts of bids and responses in our chosen systems, and then when it comes to the much more uncharted world of defence, we feel lost without strongly prescribed, objective meanings for the plays we make and the responses we receive!
I don't have a perfect, systemic answer, but what I would say is: think about what information you NEED from your partner, and then about the ways he may have at his disposal to communicate that information. After the dummy is laid down, you and your partner will probably have some idea what course the game is likely to take thereafter. If this suit is a non-starter, then watch partner for an indication of which suit he'd like you to switch to. If you're winning the first trick and hope to carry on leading it, watch partner for negative or count signals so you don't get trumped when it's most inconvenient.
Long story short: it is context-dependent. So don't agonise about what a single played card means in isolation (that way madness lies - it could just be a singleton after all!) Instead ponder what information would be useful to you to help you defeat the contract, and watch partner like a hawk for any unusual plays that may be trying to give you that information. As time goes by your partnership will hopefully develop a rapport that will enable you to communicate effectively in various situations - and that's the route to playing great Bridge!