How to check request as result of 301

Assuming that the user lands on a BigIP virtual after being redirected.

Normally there is nothing in the request to indicate how a user landed on a site. If however, the server which sent the redirect is being proxied by a BigIP, you can intercept the redirect and include a some tracer. The best way here may be to include a special query parameter. When the bigip receives the request, it will detect the parameter and act on that. The bigIP can also strip the query parameter before sendging to backend so application does not see it.

HTH