I find it extremely difficult to explain how to use the Search function on the SDS series to search out P25 control channels. This mainly stems from the fact that none of my Custom Search options are the default, so I would have no idea which one to tell somebody to use.
In general though, the Search function is going to be done via Menu (red button on left side of scanner) --> Search For.
Here is what I do -- I choose Custom 9 for any search ranges that I know I might change over time.
Menu --> Search For
Select Edit Custom
Scroll down to Custom 9 and press down on the volume knob on top of the scanner
Select Edit Srch Limit
Type in 854. and press <E>
Type in 862. and press <E>
Then press the Menu button (red button on the side) until you back out to the point where you see Search For: Custom Search. Select Custom Search.
At that point, make sure 9 (and only 9) is showing up at the top of the screen under "Search". If other numbers are showing up as well, push each one of those numbers individually to disable those custom search options 1 through 8 -- until you only see "9", which is Custom 9, active.
At that point you should be scanning between 854-862 mhz. Then, it's going to pause on any birdies / interference / P25 / EDACS / DMR or any other active frequencies.
Although, if you were asking about doing this because I had asked you if you'd check for active P25 control channels in your area, you can probably disregard. The stupid SDS scanners do not show all of the relevant information about a P25 control channel that we would need. You have to jump through 50 hoops in order to be able to see all of the really relevant information. So it's probably pointless to even do this (if you were doing it to help me out).
If you do decide to do it, you'll specifically be looking for frequencies that:
a. displays P25 NAC ???? -- where ???? is a hex System ID
b. displays SysID: 1F6h or 1F7h
If you run across any that meet both A and B, please note the frequency and report to me (via private message or via the AEP P25 forum thread) the SysID and NAC shown.