The rules don't specify how noticeable spell-like, supernatural, or extraordinary abilities are in general. There are specific abilities that have text with such ramifications (e.g., Cackle hex), but no general rule. So, as with so many things, the GM is left to decide.
I would allow the use of telekinesis as a spell-like ability without breaking Stealth, but another GM might not. I would not allow magic missile to maintain Stealth without a sniping roll, but another GM might allow or completely disallow.
With no rules to specify the mechanics for activating an SLA, it's up to you to decide (or your GM). I assume the activation is easy because, unlike spell casting, SLAs don't provoke. So no concentration, no gestures, no speech, etc. (although I usually do have most things make some kind of gesture, even if it's only to gaze malevolently at their target - I make it clear that it's not an actionable gesture, so there won't be any obvious way to grapple, etc., to prevent it).
Of course, some SLAs produce highly visible or audible effects, but some don't. For the latter, it might literally be impossible to detect that the creature did anything at all.
Spell-like abilities do provoke and can be disrupted. (I forget this at least once every other session.)
My general rule of thumb is that if a spell-like ability targets or affects an unwilling non-ally then it qualifies as an attack, and would therefore cause Stealth to 'break'. Otherwise, as Blahpers and DM_Blake noted, it is up to the GM.
Where does it say the b?
Using a special ability is usually a standard action, but whether it is a standard action, a full-round action, or not an action at all is defined by the ability.
Spell-Like Abilities (Sp): Using a spell-like ability works like casting a spell in that it requires concentration and provokes attacks of opportunity. Spell-like abilities can be disrupted. If your concentration is broken, the attempt to use the ability fails, but the attempt counts as if you had used the ability. The casting time of a spell-like ability is 1 standard action, unless the ability description notes otherwise.
Using a Spell-Like Ability on the Defensive: You may attempt to use a spell-like ability on the defensive, just as with casting a spell. If the concentration check (DC 15 + double the spell's level) fails, you can't use the ability, but the attempt counts as if you had used the ability.
[http://paizo.com/prd/combat.html#use-special-ability]Link[/url]