I know that it's possible to create dependencies of mods on ESM files. But what's the best way to create a dependency on a mod that doesn't have an ESM file, such as SI?
The main reason I ask is I want to make sure the mod I and others are working on cannot be used if SI is not installed.

In my http://www.tesnexus.com/downloads/file.php?id=16308 mod, the game does a CTD when the player nears the towers and they do not have SI installed.

It may be because I am using SI creature resources in the outside cells. Since they do not exist in the vanilla game, the game Crashes.

What does the game return if you look for a SE only Global that does not exist ? 0 , -1 or Crash -> if it returns 0 or -1 then you can check any of the SEPieces since they are all above 0 and if the return is greater then 0 then its installed and if not then its not.

if using OBSE - try

if !(IsFormValid somePersistentRefEditorIDfromSI)    messagebox "mod XXX requires SI"    ; stopquest etc    returnendif

EDIT : of course crashes will still occur as explained by WillieSea as long as SI is not installed and your mod is

Add ShiveringIsles.esp (empty file for .BSA auto-load) to your plugin's master list with TES4Edit. An .esp can have another .esp as a master, whether empty or not, and can even reference forms from said .esp as though it were an .ESM. The CS won't have it although the engine will. If you need to edit the file with the CS, "Clean masters" or manually remove ShiveringIsles.esp from the master list in TES4Edit, open and edit with the official editor, then add SI to its master list before releasing/playing it that it won't load without Shivering Isles' .esp.

The only way I can think of doing what you're after without adding the .esp master is kinda convoluted, but... By checking the status of a shared record which differs between the original and SI Oblivion.ESM files, one could script certain elements to hinge based upon the value of a 'bShiveringIsles' variable or global set via indirect detection of SI. Say PersistentReferenceX had Z coordinate value of -1234 in the non-SI Oblivion.ESM and Beth moved it so its Oblivion.ESM [SI] Z coordinate value is -4321. Set bShiveringIsles to (PersistentReferenceX.GetPos Z == -4321) would allow for 'If bShiveringIsles/Else'. There's bound to be a detectable difference between the two Oblivion.ESM's.

*installs Oblivion sans GOTY disc on a virtual machine to acquire an antiquated Oblivion.ESM for "Compare to" in TES4Edit w/o borking playing install*

Took FOREVER to filter/clean one Oblivion.ESM against another, but...

scn ShiveringIslesDetectionSCPTInt bCheckedInt bShiveringIslesInt bMessageBoxInt iButtonFloat fQuestDelayTimeBegin GameMode	If bChecked		If bShiveringIsles			StopQuest ShiveringIslesDetectionQUST		ElseIf bMessageBox			Set iButton to GetButtonPressed			If (iButton == 0)				Player.Disable ; ExitGame			EndIf		Else			Set bMessageBox to 1			MessageBox "MyPlugin.esp requires Shivering Isles! Oblivion will now close", "OK"		EndIf	Else		Set bChecked to 1		Set fQuestDelayTime to .01		Set bShiveringIsles to (TestDead.GetAV Fatigue == 160)	EndIfEnd

The SI Oblivion.ESM gives that TestDead NPC 160 Fatigue while the non-SI Oblivion.ESM gave him 170.

Set bShiveringIsles to (MQ11KvatchAllyMarker.GetPos X == 21619.564453)

would also work as the X coordinates were formerly 21592.013672, but TestDead is probably less likely to have been edited by any mods.