Give them a reason to explore
You are most likely correct that a mini-lecture on the benefits of exploring in game will not get your players to explore more (and may annoy them, as well). You telling them that exploring is a good thing will never be as good as them realizing that they need to explore on their own. But what you can do is provide opportunities.
Think about the reasons that exploring is important to a fun, successful game, and then make those reasons super-obvious in front of your players. Some examples:
- Exploring reveals nifty loot: When the players are fighting in a dungeon, have one of the adversaries pull a weapon or other equipment out of that chest that the players are supposed to search.
- Exploring saves your neck: Spring traps on your players. At the beginning, avoid causing serious harm, because you don't want to run the risk of seeming adversarial and making your players resent you for it. But once it's known that traps are a thing to watch out for, it's your players' fault for not keeping an eye out.
- Exploring reveals story: Plant clues in the dungeon. This one is a little more difficult to signpost. Maybe the players stumble upon the Evil Lieutenant packing up some plans into his desk. Maybe the Old Man in the Tavern literally tells the players they should be looking for something in the ruins.
This advice may or may not mesh perfectly into the published adventure you're running. Don't be afraid to tweak the details of the adventure to suit your purposes at the table, though. If that means adding some things into the environments for the players to find, go for it. If it means installing some extra traps, go for it. Pretty soon, you'll find them checking for secret doors in every room, and spending ten minutes disassembling all of the furniture when no one's looking.
I would say take a look at online gaming pages. Most of them have the ability for you to mark it as a custom system, and then put that it's a playtest either in the title or the body. Or they have a forum where you can say that you are seeking playtesters for your custom RPG system and ask if they are interested, and then give them the link to that game.
Also most of these have the ability to designate co-GMs so that you could see it all AND also still observe instead of actively GMing.
For personal experience there, I'm active on roll20.net. There I've done a few playtests time and again by just titling the game "Playtest-Fantasy Dragons war" as example. In the description, I describe a few details about the setting and also the game system; for example:
Fantasy setting not unlike Golarion, with the players taking the role of dragons who awaken from a long sleep and must battle an ancient foe to survive. The system itself is attribute and skill based and uses D20s for rolls. Thus a typical skill roll is D20 + attribute + skill.
Additionally I make a "Check here if you're interested in playtesting" forum post and link to the "group up" page for that game there.
The mentioning of how the system basically is laid out is exceedingly important in addition to the setting used, as players use BOTH of these to decide they are interested or not... Alongside the info that it is for a playtest, you should also make clear what they should expect.
Thus, if you expect to tweak rules every 2 sessions and also to get a report by the players who they like that, then you should include that information in the initial description of the system as that is also important information for them to know.
The above should hold true regardless of which online gaming site you would use (thus play-by-post and chat-based or "phone"-based gaming).
Best Answer
What the answer comes down to is "exercise your social networks, both online and offline." Be both looking for gamers/groups of gamers you can join and also registering your interest so that groups of gamers interested in a new player can find you.
Decide what you want to play and prepare your pitch
Do you care what game(s) you will play, can you host, can you drive to a game and if so how far, etc? You need to decide on this so you know whether "I found a D&D 4e game that meets in a library in Hoboken 60 minutes away" would be good or not. This helps you refine your pitch when you talk to other gamers from "I wanty the game" to "I'm a [new/experienced] gamer looking for [any RPG, a D&D game, prefer D&D 5e but am open to others, etc.] within [walking distance, a short drive, 120 minutes, remote only, etc.] of [where you are]. Also note what you have to provide - "I can provide a gaming location," "I don't mind GMing," "I am new but enthusiastic and really want to learn from an experienced group..." Note limitations that the group really should be aware of, like "I am allergic to cats" or "I'm a minor" or "No Republicans."
Shop it around online
Definitely use online resources.
But don't use them to the exclusion of offline resources!
Get off your chair.
If All Else Fails, Game Online
There's also a variety of play-by-post (forums, email, etc.) and play-by-chat (including virtual tabletops and Roll20) if you just really can't get people together in person; all of these have LFG sections. For more resources there, Finding online RPG players for a play-by-chat RPG Campaign?, Where can I advertise for players for my play-by-post game?, and the play-by-post and play-by-chat tags.