The closest you're going to get on a typical home ISP link is to analyze the MAC addresses of your traffic, and if the gateway changes to something else it's a sign that something has changed on the network. That could be an eavesdropper, or it could be a simple equipment changeout.
However, if the snooping is happening beyond the gateway even a sloppy snooper will remain undetected. Also, if the snooper is using a simple bridge rather than an attack based on ARP-spoofing, the only indication of their presence will be a slightly higher latency; slight enough it'll hide in the statistical noise for most home broadband connections.
If the snooping is actually an intercept proxy of some kind (layer 5-7) instead of a network tap (L2), say sniffing all of your HTTP traffic but leaving the rest alone, there is a possibility that you can detect that by watching your HTTP headers. If their DNS settings are crappy you may find some sites unavailable that really should be.
And finally, stateful firewalls do exactly this kind of thing by design. Figuring out the difference between a stateful firewall and a snooper is very, very tricky.
In short, it is very hard to detect snoopers, even sloppy ones.