This thread reminded me of a post I made that outlines what I do to evaluate optics during daytime, not just for optics but mechanics as well (say, if you were to visit a seller to try a scope before you buy it). I finally found it! https://www.cloudyni...t/#entry9313202
For ease, I will copy and paste it here, but there is some other chatter in that thread that might be valuable. Also, I didn't really talk about tree branches against a clear sky as a CA test, but that has been mentioned here, and is the most common way to see CA during the day. Move the branch(es) around the FOV to see how the CA changes from center to the edge.
Snap to focus has been a good test for me, personally. There is a lot that can be in focus during the day, so your best bet is focusing on something that doesn't have anything else near it, like a road sign that is a bit elevated above the observing position, so the sky is the background, or a roof several dozen feet away. I also like to look at areas that DO have a lot of things near each other, like grass, reeds, cattails, trees, etc. I rack the focuser back and forth and judge just how sharp the sharpest part seems to be, and how best focus drifts from one object to the next as I focus in and out. This is also a good way to see chromatic aberration. The out of focus areas will take on a tint, usually, in a doublet. My TV-85 displayed a slightly purple or green tint overall for out of focus objects, but also added a slight falsely colored rim to edges of things like grass or cattails or tree branches.
What to bring:
* Your best (or typical, I guess) mirror diagonal, and best (or typical) prism diagonal.
* Your best (or typical) eyepieces, at approx. lowest mag, second lowest mag, medium mag, and high mag. If you anticipate local seeing will be good, bring your best highest mag eyepiece as well. You can also include your favorite eyepiece(s) if you have one, even if it's in between those categories.
As far as success criteria goes, I would look for the following:
* Clear indication of focus. This is "snap focus" that people talk about. Even great optics with low SA p-v won't always OBVIOUSLY have a perfect point of focus, but you should be able to easily get something in focus without having to fuss too much with the fine focus knob. You should be able to rack far in, then rack out at a consistent rate and STOP when you think you're about to hit focus, and hit it. You won't get it every time, but I'm surprised how often I nail focus seemingly on a whim with my good optics. I just stop and bam, it's in focus.
* Impeccable sharpness at the center. We're talking "wow that's sharp" provoking thoughts. Not "that seems pretty good". This WILL depend on magnification and local seeing conditions, however, so compare it at low, medium, and high mag. You probably won't get stunning sharpness at high mag during the day with terrestrial objects; you're just too close to the ground and the air will prevent the clarity you want. Definitely try it at multiple magnifications. I would personally want a stunningly sharp center focus at a ~2mm exit pupil for daytime objects. At lower mag, almost anything looks sharp, but at medium to high mag is where you'll start to see a difference.
* Complete lack of fog or haze. At higher mags, the low contrast might be mistaken for a fog or haze affecting the lens, so use low-medium mag for this. The optics should seem transparent, with no edge brightening, no fuzziness to the image, and no cloudiness. There should be no un-evenness to the clarity of the image. If you notice any sort of contrast loss, across the whole image, or in one area, check eyepieces and diagonal for dirt/smudges. If it persists, check lens with flashlight. Likely not caused by even what some might call severe dust, but could be caused due to a residue left over from poor cleaning, or possible emerging fungal problem. I'd expect this problem to be evident just by looking at the lens and to be honest I would not expect it, period.
* No rattling or other noises in the objective. This is a mechanical thing. Some Takahashi objectives apparently DO rattle per manufacturer spec due to how they've manufactured the cells, but I would absolutely verify with the manufacturer first if you do hear such a thing, before putting any money on the table.
* Smooth focuser mechanics. No grinding, rubbing, or slipping. If you have a heavy diagonal and eyepiece, bring them and point the scope at 60deg altitude. If it doesn't slip without any tension, point it all the way up to zenith. If it starts to slip, try the tension knob. You may just have to put some pressure on it (to be expected). Most focusers will require some tension at zenith with a 2lb+ load on the back. If there is grinding or rubbing, the drawtube may be misaligned, but it might be fixable with screws on the focuser itself. This is something that would not prevent me from buying the scope after looking through it, sans any other issues, but I would probably ask for a discount considering I would have to fix it myself, and it might as well be considered defective in that state. Just because a crappy focuser is "good enough" for someone doesn't mean it's a properly functioning focuser, same as how just because mediocre or average optics are "good enough" for someone doesn't mean they're actually good optics.
* Smooth dew shield mechanics. It should stay in place even when approaching zenith, but some will slip. Wouldn't be a deal breaker for me as you can probably put some sticky tack or something behind the dew shield once you've extended it, and remove it after the observing session, if it slips easily. It is also possible to have a silicone ring created that would slip over the dew shield onto the OTA, and you'd roll it up behind the dew shield as you extend it, then roll it back down onto the OTA to retract the dew shield. If there is a locking screw then just make sure it works and doesn't mar the OTA if you accidentally try to slide the dew shield down with the screw engaged.