How can you determine the Lotus Domino server install type, so that you know whether the installation type supports running a Domino cluster?  

From the Domino server console, you can issue the following command:

show config installtype

The result will looking similar to:

INSTALLTYPE=4

The variables for installtype are outlined below:

0 = Designer Client
1 = Administration Client
2 = All Clients
3 = Domino Mail Server
4 = Domino Enterprise Server
5 = Domino Application Server (pre R6.5)
5 = Domino Utility Server
6 = Notes Client
7 = Notes lite
9 = Unknown, set by default at start of installation process, set for shared install

Therefore, the server above where it read INSTALLTYPE = 4 is an Enterprise Server.
 
Note: In Release 5, Domino clustering was only available in the Enterprise package. New changes were made in Domino 6 to include clustering in the Lotus Domino Utility server.