Yes, for PowerServer 2021 and newer read this: https://docs.appeon.com/pb2021/powerscript_reference/ch02s04s461.html

For PowerServer 2020 and older, read this: https://docs.appeon.com/ps2020/workarounds_and_api_guide/AppeonExtFuncs_object.html#of_getclienttype_function

Here's how I do this:

// Local External Function

// OKnight, 24-MAY-2006.  Migrated to PB v10.5.
FUNCTION Ulong GetModuleFileNameW ( Ulong Handle, Ref String ExePath, Ulong Size ) LIBRARY "kernel32.dll"


// corp_n_cst_appmanager.ue_post_logon()
// ---------------------------------------------------------
String        ls_toolbartext, ls_executableFullPath

// Initalize
ll_size = 200
setNull(ll_handle)
ls_executableFullPath = Space(ll_size)

// Get the full path to this current executable
ll_RC = GetModuleFileNameW ( ll_handle, ls_executableFullPath, ll_size )
ll_length = Len ( ls_executableFullPath) -  LastPos ( ls_executableFullPath, '\' )
is_executableName = Lower ( Right ( ls_executableFullPath, ll_length ) )

// At this point if I'm running from the IDE the is_executableName = 'pb190.exe'

PB2021/PS2021 does have a built in powerscript function for that. 

Earlier versions do not, you can create your own function and have 2 version of it.  The PS version you deploy only in PS only pbl that is listed in front of all other pbls:

C/S version:

global type f_ispowerserver from function_object
end type

forward prototypes
global function boolean f_ispowerserver ()
end prototypes

global function boolean f_ispowerserver ();RETURN FALSE
end function

powerserver version:

global type f_ispowerserver from function_object
end type

forward prototypes
global function boolean f_ispowerserver ()
end prototypes

global function boolean f_ispowerserver ();RETURN TRUE
end function

Have you tried function IsPBApp? I don't know what it returns with PowerServer.

In PB2021 there is also a function IsPowerServerApp!