The best I've come up with so far is to have two test systems (typically VMs) that use adjacent IP addresses and adjacent MAC addresses, and test both inbound and outbound to/from those, blindly trusting/hoping that hashing algorithms will
probably exercise both paths.
If the goal is to test that traffic *is* being distributed across multiple links based on traffic headers, then you can definable roll your own. I think the problem is orchestrating it (feeding your topology data into the tool, running the tool, getting the results out, and interpreting the results etc).
A coupe of public examples:
If you do roll your own, you need to taylor the tests to your topology and your equipment. For example, you can have two VMs as you mentioned, each at opposite ends of the network. Then, if your network uses a 5-tuple for ECMP inside the core for example, you could send many flows between the two VMs, rotating the sauce port for example, to ensure all links in a LAG or all ECMP paths are used.
It's tricky to know the hashing algo for every type of device you have in your network, and for each traffic type for each device type, if you have a multi vendor network. Also, if your network carries a mix of IPv4, IPv6, PPP, MPLS L3 VPNs, MPLS L2 VPNs, GRE, GTP, IPSEC, etc. The number of permutations of tests you need to run and the result sets you need to parse, grows very rapidly.
Cheers,
James.