I have made a big model, but I have one bad polygon with 5 vertices. It's number is something like 483, But I have no idea where it is. How would I find the bad one quickly?

Go into Polygon mode (or whatever you want to call it) and type the number of the polygon in the text box highlighted below:

Thank you, that was it.

even easier and finding ALL bad polygons (so not important if they have 5 or 6)
go on select (is on Marth photo) then n-sided polygons and then press 5 or more. Then you will get everything at once