In a DOS box, run:

mode com1

If you get "Illegal device name - COM1" then the comport isn't enabled.

Better yet, run:

debug

d40:0

The first line will list the COM ports on the left side (right side is parallel ports) of the hyphen (in reverse byte order). For example, I get:

0040:0000  F8 03 F8 02 E8 03 E8 02-BC 03 78 03 78 02 C0 9F

Meaning, I have COM1 enabled with the address of 03F8, COM2 at 02F8, etc.
If you get 00 00 for COM1, it is disabled.