Before I buy , i want to make sure: will a 430exII fire by "seeing" the light burst from other flash sources? and if not, what canon speedlite can do this?
No, the 430EX II does not trigger by generic photo slave.
However, it can be remotely triggered using a compatible Canon system flash trigger. Those include the ST-E2, or a 580EX/EXII, 600EX flash units.
In addition, some of the newer Canon cameras, supporting what Canon calls the "Integrated Speedlite Transmitter feature" (popup flash) can also trigger this flash, Those include the 60D, 70D, 7D, and the T3i(600D). There may be some even newer bodies that support this as well but not older bodies like 40D, XTi/XSi.
No, the 430EXII does not have a "dumb" optical slave built in. None of the Canon EX speedlites do—the only "slave" capability that's built in is for use in Canon's proprietary light-based triggering system. The 430EXII can act as a slave to a Canon master unit (pop-up flash in a 600D or later dRebel, 60D or later XXD, 7D, and 1DX, ST-E2, 90EX, 550EX, 580EX, 580EXII, or 600EX).
To get "dumb" slave capability, you have to add-on an optical slave unit, and again there is something odd in the Canon EX circuitry that means a great many of the more generic add-on optical slaves may not work. The green-based Sonia slaves are known to be compatible with Canon's EX speedlights, and they come with male PC or 1/8" miniplug connectors (see: this Syl Arena blog article on Canon speedlites and optical slaves).
Which means to use an optical slave on a 430EXII, you also have to add a hotshoe-to-sync port adapter. At which point, the cost may well be the same or more than cheap radio triggers, like the Yongnuo RF-603II (or whatever the cheapie du jour trigger is when you read this).
Speedlights that have a "dumb" optical slave built-in, however, do exist outside the Canon lineup. Nikon's SB-700 and SB-910 both have "SU-4 mode" which is a built in "dumb" optical slave mode. Most plug-in studio strobes have optical slave sensors. And current Godox, Lumopro, Yongnuo, Metz, and Nissin flashes also have dumb optical slave modes built in that can ignore a pre-flash. In fact, most 3rd party speedlights have two dumb optical slave modes: one that fires on the first flash it sees, and one that fires on the second flash it sees (so your "master" flash can be in TTL and fire a metering pre-burst without triggering the slave flash until the main burst); these are typically called S1 and S2 modes (although I believe Nissin calls them SF and SD).