The Ripple account reserves are documented on the Ripple wiki.
The trust line reserves are only for trust that you set. Others cannot cause your reserves to increase by adding a trust line to you.
The client warns you when you are bellow twice your current XRP reserve limit by placing a warning symbol beside your XRP summary (located at the top right of the client). Wheather the warning is shown or not, hovering the pointer over the XRP summary will provide a "tool tip" with the current reserve requirements.*
If an action would put you under the reserve requirements (such as sending too much XRP, creating a trade offer, setting a new trust line) the action is refused with an error. When below the reserve requirements you can still do other transactions so long as you have enough XRP to pay the 0.000010 XRP transaction fee.
- Note:There is currently a client bug that causes the reserve requirements shown to be twice the old levels, about eight times the current reserve requirements.