10) How much do they flow - The first and sometimes the only question asked.
(This is a loaded question. You will find out why this is #10 and #1 as you continue reading)
9) Intended use/type of head - what head do you really need,23 deg or maybe a 10 deg head.
8) Valve seats - valve job 45* or 55* / size 2.080 or 2.200 / seat material cast iron, powder metal, alloy 25 copper or A3
7) Guides - Mag material or something cheaper, 5/16 or 11/32 maybe 8mm
6) Valves - If the heads come with valves what did they come with? Import garbage or some good stuff? Note: even some good name brands are branding junk valves. What size and angles?
5) Spring info if they are assembled heads, will they work with your cam
5, 6, 7, and 8 above are all related to 9 intended use. If you are building an engine for a street machine or a low RPM bracket car cheaper parts will work. Import valves can work if they are good quality and cast iron seats will also work fine.
For engines that will be abused good parts will last longer and will prove to be cheaper over time due to less breakage and longer life. Alloy 25 copper seats can be costly up front but will last much longer and will also help keep the valves in good shape as well. Same goes for Mag guides which will last much longer than all other soft guides but cost more on the front end.
Now for the most important ones:
4) Port volume - what is the proper size of the port? Are they too big is a common question but usually HRD finds the ports are rarely big enough (another intended use question).
3) Chamber volume/size - is the cc correct for your application and is the bore size correct for the chamber? If your cylinder head builder does not fundamentally understand the relationships here, there is horsepower sitting on the table for sure.
2) Intake Mid lift flow - a question that nobody asks, you need good mid lift not just peek numbers, .500 is an extremely important number.
1) Exhaust flow - the most overlooked question. Whether big displacement, blown, turbo, or nitrous, a bigger engine typically requires a precisely designed and tuned exhaust port.
In speaking with your cylinder head builder, make sure they understand the importance of exhaust flow (.300 to .400 transition very overlooked numbers).
Believe it or not, at specific low lift flow ranges you actually do not want the maximum exhaust flow possible. This is typically dictated by the mechanical dynamics occurring between the piston and valve opening sequences in order to maximize the amount of intake charge being drawing into the cylinder.