The only way to tell is to try it out in person. Everybody's eyes are different. Particularly if you wear glasses, some displays are not good when your glasses require you to be further from the display. On most electronic viewfinders, there is typically a dial to change the diopter of the display. You want to dial in an appropriate diopter so you can see the display. Some people who normally wear prescription eye glasses can dial in the diopter so that they don't need glasses to use the viewfinder (of course if they hand it off to somebody else, it will appear to be out of focus). Note, if you go for the original E-m1 mark I, you don't want to dial in strong diopter values as there is a common reported problem that certain diopters can cause the viewfinder screen to burn out if exposed to the sun.
I tend to be of the opinion that if you use a store to try out a camera in person, you should buy it from that camera and if a salesperson helped you, buy it from that person so they get the commission. After all, the store helped you and gave you the value of trying it out in person, which should counter getting the camera elsewhere on the internet at a lower price. If nobody buys from the store, sooner or later, it will go out of business.
There are generally 3 types of viewfinder, each with pluses and minuses:
1) The TFT LCD viewfinder. This is used in Olympus E-m1 mark I/II, E-m5 mark I/II, and the original E-m10 mark I bodies as well as the VF-2 external viewfinder on older Pen bodies. I think the E-m1x also uses a TFT LCD viewfinder. A lot of people feel the colors are more 'natural' with a TFT viewfinder. The refresh rate of the viewfinder can be cranked up fairly high.
The major downside to a TFT LCD viewfinder is when you shoot using polarized sunglasses. Unfortunately, due to migraines, I need to wear polarized sunglasses all of the time when I'm outdoors in sunlight, so it is a hot button issue with me. In particular, on the Olympus cameras, if you shoot in landscape (horizontal) orientation (i.e. the normal way people shoot), the viewfinder can either be completely opaque (VF-2, E-m5 mark II) or there are bands on the viewfinder where you cannot see the image with sunglasses on, and bands where you can see the image (E-m1 mark I/II, E-m5 mark I). I've found that if I use the viewfinder, I can generally frame the shot with the area that I can see, but it is annoying, and I wouldn't be able to do detail work like manually focus. If I switch the camera to portrait orientation, the viewfinder is completely usable.
The rear display of most cameras these days tends to be TFT LCD, but the orientation that is problematical with polarized sunglasses is portrait (vertical). In the past, the E-m5 mark I had an OLED rear display, but generally the cell phone makers consume the worlds supply of larger OLED displays, so it is rare for current generation cameras to use them.
2) The OLED viewfinder. This is used in the Olympus E-m10 mark II/III, Olympus Pen-F, and the mid/upper Panasonic bodies (G85, GH5, GH5s, G9, etc.). Unlike TFT LCD viewfinders, the OLED display can be used in either orientation with polarized sunglasses (typically one orientation is darker with sunglasses, but there is no visual disruption). OLED monitors also tend to have a wider angle of view than TFT LCD monitors, though I'm not sure it matters as much for a viewfinder as compared to the rear display.
IMHO, the major downside to OLED displays is they tend to saturate the colors more. Now, a lot of people like the pop of a saturated display, but I find the pictures will appear to be duller when viewed on a computer if I shot the scene with an OLED viewfinder and adjusted the photo in the camera for the viewfinder. I now mentally tone down the image to allow me to get a better perspective on what the final image will look like (and/or just boost up the levels in post processing).
I've seen complaints that the refresh rate for most OLED viewfinders is lower than a TFT LCD display, and that for some people, this low refresh rate can lead to migraines. I've heard that the Panasonic G9 has a higher refresh rate, so it probably depends on the camera.
Another issue to consider is historically, OLED monitors seemed fail earlier than the same generation TFT LCD display. In particular, over time the LEDs start producing less light, particularly the blue LEDs, which can mean that before it becomes unusable completely, the color rendition may be off. Now, I'm not sure whether this is an issue with cameras, as it may be based on the amount of time the display is on. People who take relatively few pictures, might not use the camera enough for it to start failing. Pros shooting for a living and amateurs that blast out photos tend to replace their cameras every few years, and may not see the failures (of course this might be an issue if you buy a used camera from somebody who did shoot tens of thousands of images).
The power usage of an OLED tends to be different from a TFT LCD. The TFT LCD tends to be more consistent since the main power draw is the backlight used to illuminate the screen, while on an OLED, a lighter scene will consume more power than a darker scene. I'm not sure whether this is reflected in how long batteries will last before being depleted.
I bought a Panasonic G85 specifically because it had an OLED viewfinder (and was splash proof and had a sensor shift stabilization). After buying it, I found while the viewfinder worked great, I really preferred the Olympus menus and shooting parameters, so I eventually bought a refurbished Olympus E-m10 mark II to use as a good weather camera to use in bright sun.
3) Field Sequential. These tend to be the smaller displays used in Panasonic GX cameras (and LX100). Unlike OLED/TFT where each pixel consists of 3 LEDS (red, green, blue), a field sequential display uses a single light that pulses red, green, blue and depends on your eye's persistence of vision to merge the colors together. Some people are more sensitive, and field sequential displays can have a rainbow effect if the eye or camera moves rapidly. Another downside is just the smallness of the display.
