I'm going to cut to the chase here. When you have two curling stones and wish to test them for the aforementioned "speed" & "curl", they need to be delivered in the manner of a curling shot. And when you do that, you must consider three factors, speed/velocity, line & rotation. In other words, the stones need to be delivered with the same velocity (or known velocities if not the same), down the same line, with the same amount of rotation. If you do not control those three factors, the results could be skewed and you might in the end (pun intended) miss some shots as a result.
Laser timers are a wonderful aid to determine velocity. The holder of the brush can determine line and of course you can count rotations by observing the handle.
If two stones are delivered with the same velocity, down the same line with the same amount of rotation and they come to rest in the same location, voila, you likely have a pair of matched stones. If they do not and this is borne out time-after-time, then the opposite may be true. Take note of the words "likely" and "may" in the preceding paragraph. There are no absolutes with stone matching, only degrees of reliability!
Strangely enough, if your suspicion is that you don't have a pair of matched stones, that doesn't mean your world is coming to an end. I know curlers who relish the fact that two stones are not matched and the team uses the differences between that curler's two stones to their advantage. When I coached Team USA (m) at the World Curling Championships in Kamloops, our third knew he had a "cutter" (a stone that curled more than the other). We saved it for his second shot on ends when we had last rock and buried it. The opposing skip knew we had found a cutter and could not follow it. It ultimately won the game for us!
I say this to counter any argument that to not have a pair of matched stones is tantamount to being charged with a criminal offence known you are innocent. But, that said, if you're not comfortable with an unmatched pair of stones, you need to inform your team of that and enlist their aid in finding two that are matched using the method described above.
One of the most high profile curlers in Canadian curling history was obsessed with finding a pair of matched stones in the pre-game warm up. So much so that his/her teammates spent the entire time in this quest. In speaking with a teammate of this aforementioned curler, he/she confided that sometimes the team was pretty sure the stones the skip decided to use were not matched, but the skip thought so and that's all that mattered! Back to those r/s's!
Start to get to know them from stone-to-stone at your home curling facility and in facilities where you will compete. Record the stones and their velocity and curl characteristics. Most teams who are serious about their performance have a "rock book" which contains the information re. stones that the team feels are "worthy of note" (a diplomatic term replacing more common terminology like "pig" and "dog").
Stones of course have unique identifying features. First, the "cap" is coloured. On that coloured cap are numerals to identify the sheet of ice where the stone resides and the number in the set of eight. Sometimes the caps are replaced (read "exchanged") with the cap from another stone. In that case, if the stone is turned over to reveal its r/s, you might notice an etched serial numeral near the bolt hole in the centre of the stone. Some "rock books" refer to stones using that system as opposed to colour, sheet location and number in the set. If the sheets at your curling facility are lettered A,B,C,D etc. then the letter is usually found at the edge of the coloured cap at the base of the gooseneck portion of the handle. The "set numeral" is usually at the edge of the cap on either side.
Begin to turn stones over to examine the r/s of the stones you're likely to use in the game. Ask yourself these questions. Are the widths of the r/s's the same? Are the diameters the same? When I run a bare finger around the inside and outside edges of the r/s's, are they the same? And lastly, when I feel the r/s's, is one smooth and the other "pitted"? If you get some definite "no" responses, you might be suspect of the stones from a match perspective. But even with those suspicions, deliver them according to the method described above to know for sure and trust what you see!
A "dull" r/s produces a stone that will be relatively "quick" but "unpredictable" in curl. A stone that has a "defined" edge to its r/s, especially if it's the outside edge may not be quite as quick as the previous but it will "finish" (i.e. curl, especially near the end of its journey).
On occasion, you will see a curler take his/her two stones and place them together in line with the center line of the sheet. Then the stones are pushed to get them moving in tandem. If the stones remain "frozen", the velocities of the stones may be the same. I say "may" because the stone that's behind might be faster and as a result will be pushing the slower stone. The only way to control that possibility is to repeat the process with the stones in reverse positions. Simply moving them in the opposite direction accomplishes that task. Although this method does give you some information, it's no substitute for actually delivering the two stones, again, as described above.
One note of caution be I close. Don't become so obsessed with "rock matching" that it gets in your head. If you think the stones you're playing are not matched and that's why you're missing shots, that indeed may be the case but remove yourself from the equation first. Be honest! Check with the teammate holding the brush. If he/she says you're missing the brush or if your brushers look you in the eye and tell you you're light/heavy, be accountable. You're the problem, not the stones!
And if you're a junior athlete, please make sure your coach is involved in the rock matching process. Trust your coach!