You can't be arrested without probable cause. It's one of your rights. In many cases, officers need arrest warrants, though they can make on-the-spot arrests when probable cause is clear at the time -- such as when you're pulled over on suspicion of drunk driving and the breath test puts your blood alcohol concentration at 0.10.
It is important to note, however, that exactly what constitutes probable cause isn't specifically defined. It can be fairly vague, and many different factors can play into it. The stipulation is just that the office has to have some reason that is more than a hunch.
Some examples of things that can be used as probable cause include:
-- Sensory details like sound, sight, and smell.
-- Statements taken from witnesses or victims.
-- Statements taken from police informants.
-- Details that police recognize due to their own expertise. For example, an officer who works with gangs may see known gang signs, or an officer who works with burglaries may recognize specific tools used in recent robberies.
-- Circumstantial evidence, like a broken window, that connects to a crime indirectly.
These are just a few examples, but they show that probable cause for an arrest can come from many different places. Once the police have enough evidence and probable cause, they may get a warrant or carry out the arrest.
The important thing to remember is that probable cause is not enough for a conviction. It just means you can be arrested as they sort out the case. You're not guilty of anything yet, and your guilt must be proven beyond a reasonable doubt. Be sure you know your legal defense options.
Source: FindLaw, "When is an Arrest a Legal Arrest?," accessed Feb. 16, 2017