How soon after filing I-130 would a red flag show up when you try to cross the border? Is it after they've cashed the check, or how do the officers at the airport know that you've filed I-130? I'm just trying to decide when to file it. Thanks in advance!!!

It's entered into the system not long after the check is cashed. It's in when it shows up on the CIS website. From Canada there is a lot of crossing back and forth though even with a pending I130, as long as the person who is crossing can show ties to Canada and intention to go back.

We are talking to different branches of Homeland Security which DO NOT share a computer system. Therefore Border&Entry Patrol cannot see whether a person trying to enter the U.S. has been petitioned for an immigrant visa. At least as far as I know this is the case. They only see your previous entrances to the U.S. not the data in USCIS' computer system. As long as they have no reason for suspecting that you might stay in the country they will let you enter- at least when you are from a visa waiver country. To apply for a tourist visa on the other hand would not be a very good idea. And of course there always is the chance they might turn you away and that would mean you would have to file a waiver once your immigration petition is at the consulate.