MITM.watch is a website that attempts to determine if your TLS connections are being intercepted. The technique the site is using is based on a study by Zakir Durumeric, et al which suggested identifying TLS interception by looking at
. . . mistmatch[es] between the HTTP User-Agent header and TLS client behavior. We characterize the TLS handshakes of major browsers and popular interception products, which we use to build a set of heuristics to detect interception and identify the responsible product.