Matching screw sizes and types with bit sizes and types?

[--tom]

How can i tell off-hand what size(no 1 no 2 etc.) and type (philips or pozi) a particular screw is?
eg, i was under the impression that i was working with some no 2 philips head screws and it turned out that they were no 3 Pozi head - there was no indication on the box as to what type of screw it held.
So how would i know immediately how to match, say, a no 2 Pozi bit with a no 2 screw?
Also, how would i know which size the bits were unless they were obviously labelled? Trial and error doesn't seem to be working.
I ask this because some bits seem to be the correct type and size and yet cam out and chatter when trying to drive the screws home.
Could it be my cordless is the problem?

 

[2scoops0406]

Most bits are labelled PH1 PH2 PZ1,2,3 etc (Phillips Pozi etc) as for telling which is the right one, it's pretty obvious when you put the bit into the screw. You can immediately tell the difference between phililps and pozi's, pozi's have a second cross at 45 degrees to the main one, philips don't. Also, I noticed a big difference when changing from normal steel bits to these

 

[big-all]

when your talking wood screws about 95% are no2pozi
in general no4s are pz1 no6 to no10 are pz2 and no12 and no14 are pz3

and when it comes to metric equivilent pz2 covers virtualy all

if your screw falls off or at least isnt a good fit on the bit its the wrong bit

 

[timtheenchanter]

If the screwdriver fits???

 

[--tom]

thanks for your replies to my question.