Yup, for checkpoints "Plain Fields/View Doctrine" applies... they have no right to ask you to open your hood/trunk/glove box.
On how to tell them, diskarte na yan as the situation varies.
Question, if the type car your driving has a "alert" on it (i.e. it resembles a stolen or hot vehicle that they're looking for), would it qualify as a reanable cause to have the hood opened. Or can they ask to check for papers of the car?
siguro they should at least inform you that there was a tip that a car has been stolen that resembles the car you are in. Kasi in many cases held, may mga tips lang for example na a person is on a JAM Liner bus headed to batangas na may dalang drugs. Ito na yung tinatawag na "exigencies." Kung kukuha pa sila ng warrant, aba eh makakatakas na yung pasahero. So pwede sila magconduct ng warrantless search.
Usually di naman sinasabi ng pulis if your car resembles to the one stolen or whatever crimes related, like in our case, huli na sinabi. Wala kang magagawa if they force you to open your trunk just keep your eyes open and pag may kilala ka in the higher ups report mo yung pangalan ng pulis na yan. Much better if you have a tape recorder handy just in case...