There is no real prolog in IL code because it does not need to manage the stack, save clobbered registers, or do any other standard bookkeeping necessary in the native code.
However, the bytecode itself is preceded by the method header, and those have a limited number of possibilities. From the book .NET IL Assembler:
Method Header Attributes
The RVA value (if it is nonzero) of a Method record points to the
method body. Two types of method headers—fat and tiny—are defined in
CorHdr.h. The first two bits of the header indicate its type: bit 10
stands for the tiny format, and bit 11 stands for the fat format.
A tiny method header is only 1 byte in size, with the first two
(least significant) bits holding the type—10—and the six remaining
bits holding the method IL code size in bytes. A method is given a
tiny header if it has neither local variables nor managed exception
handling, if it works fine with the default evaluation stack depth of
8 slots, and if its code size is less than 64 bytes. A fat header is
12 bytes in size and has the structure described in Table 10-1. The
fat headers must begin at 4-byte boundaries. Figure 10-4 shows the
structures of both tiny and fat method headers.
So if you take some .NET binaries, look up method RVAs in the metadata and go to that RVA in the binary, you can collect some patterns of headers and use them to find bytecode in the binary. (although I would suggest just using metadata in the first place - it lists locations of all legitimate methods in the binary).