In a group ride when I'm the one pulling, I often unknowingly leave my friends behind until I turn my head and see them in far distance, then I had to slow down.
Is there a way to know that they are still tailing me without turning my head every few seconds? like, does it feel different somehow? or maybe there is a way to peek that I didn't know of?
A group riding together and cooperating will ride to a set speed, so you'll be surging if you exceed that speed for long. A simple wheel-magnet speedo and bike computer helps show this on your bars without being big or expensive.
An ad-hoc group will ride with varying speeds depending on who's in the front and whether they are trying to make a break away or if they're just sick of waiting for the rest of the group to cycle through the paceline.
Some riders need short pulls and some can cope with long pulls at the front. Separately some people need short rests out of the wind, and some people require quite long periods behind someone else to regain their pace.
The best groups work together for the end goal which is a good training ride.
In racing, your team is your group and the other teams are NOT your friends so they're working for themselves. A race peloton is therefore an adhoc group of teams who are not working together.
If you face some decent climbs in your route, then groups tend to fall apart as everyone climbs at their own pace. For training and bunch rides, stop at the top and let everyone get there, then rest till the last arrival is ready to carry on.
Otherwise the first arrivals are rested while the last people are shattered and the group breaks up or is held back.
Same goes for downhill - strava segments work on segment time, not first-down time :)
More thoughts - A shoulder check every half minute should show you if the rest of the paceline is there still.
If you don't have a speedo, riding to a set cadence might be enough. Try and memorise the speed of your feet while in the paceline, and simply maintain that rhythm when you hit the front of the line. Sing a song in your head of the right speed, or count off like marching if that works for you.
Getting expensive - there are race-radios like the pros wear. They are essentially Personal Shortrange UHF radios with an earpiece and a small transceiver in a pocket. The aerial/headphone cable runs up the back and is often taped on. Some new ones are small enough to work with the helmet. These talk to each other in the team and to team car, who then relays messages onwards with a second radio or maybe a cellular phone. Some are even bluetooth based and just associate with one~four other units. Motorcycle riders might wear these under their full face helmets.
On a sunny day you can often see the shadows without moving your head, so long as the sun isn't in front of you or too high. I sometimes pick up wheel suckers when commuting and while my commuter bike have mirrors its possible to ride too close for the mirror, but the shadows on the road work well if it's a sunny day.
The mirror itself is a solution, but one that presumably doesn't appeal.
I'm not normally so tucked, even in the drops, but sometimes you can dip your head and look between your legs.
The garmin radar wouldn't help, but it's possible that some of the other rear alert systems could be set up - certainly rearview cameras would work.
You could always ask them to let you know if you drop them, by shouting.
A bike mirror allows you to see what's behind you while you're facing ahead.
If you're concerned with the width of your bike during group rides, you can opt for a helmet or frame-mounted mirror rather than a handle-mounted mirror.
Especially for more casual riding, motorcycle intercom systems can be a nice addition for group rides.
It will allow you to talk easily with the other riders, who can then notify you if you are getting too far. And typically the range is only up to about 100 meters, so eventually you'll notice when you lose the signal.
A bar-end rear view mirror should do the trick. Perhaps something like a "Sprintech Left Side Mirror".
Low profile, and easy enough to peek behind to check on the rest of the paceline..
