Dropping names is just as much an art as registering names, I think. I do sometimes look at the whois checks, but find them pretty much useless. They often increase when a name is expiring, and I have never been able to make a connection between volume of whois look-ups and increased sales.
Future domains are easier, as I can just look and see whether the future I envisioned is closer to reality or has simply fallen off the charts.
If a domain has enough parking to revenue to cover renewals, that's a no-brainer. Why on earth would anyone drop a name it costs nothing to keep? But since most of my domains don't make that much, it's of limited use.
Inquiries? One year is not enough time for most names. I have picked up many bargains from impatient domainers who are expecting regular offers on brandables and names that simply will not get regular offers. A name might take 5-6 years or longer to sell. It might not get a single inquiry for all that time until it does sell. A lot of names that go on to sell for 4 figures (and more) have never had previous offers.
If a name has gotten no inquiries over several years, and it's not a future name waiting for its time to come, I do take a closer look at it. This I think is where it's difficult. No parking revenue, no inquiries...what to do? The following list helps:
1- What is the maximum potential for the name? Is it something that might get $500-$1000 maximum? (Like maybe the name of a bar or cafe.) Then maybe the reward isn't enough to cover holding costs for these names. I'll drop it. Could it have the potential of being a big outlier sale? Then maybe I'll keep it.
2- Is it registered in other tlds? If so, are they being used as websites, or all they all held by domainers? If the alternate tlds are held by end-users, it may be a keeper. If all the tlds are held by domainers, it might be one of those mass-delusions domainers suffer from. A bunch of names that will never find an end user. Still, if I hold the .com, I can at least try to pawn it off on a reseller.
3- Check the keywords for usage. Are they commonly used by established companies - preferably larger companies? I'll keep it even if I have gotten zero inquiries. Someday that ship may come in.
4- Is it just a good name? (Here's the art part.) I have names that I know will make a good business. I can imagine the business, and I could see wanting the name if I were in that business. I try to look at the name as impassively as possible. This is not easy, as I obviously liked it enough once to buy. Yet, each year our skills improve, so I may see flaws that I missed a year ago. Also each year times change, and the name may not look so attractive anymore.
5- Am I just sick of looking at the name? Then I get rid of it.
6- These decisions are all influenced by how much cash I have on hand as well. If I feel flush, I renew names I probably shouldn't. If I am tight, I get ruthless. I have dumped names just to see them sold by another domainer. Where I can, I ask them how they sold it. So at least I gain a learning opportunity from the name.
The last point is that some names I drop will be picked up and sold on to end-users. That's life. At the same time, I pick up names here and there from domainers who have given up on them, and sell them on myself. It's nothing to get upset about. If I were to never drop a good name, then my renewal costs would put me out of business. You gotta trim, and there's no way to get it 100% right.