Observing and monitoring trichomes is a very good way to help determine when to harvest a marijuana crop at the optimum time, for more info see this.
Unfortunately, trichomes are too small to be observed by human beings with the naked eye. Therefore, a tool to magnify the trichomes is needed.
Here is a list of the various magnifiers that the author has tried as aids for observing and monitoring trichomes. There is no single method that will work best for everyone.
In general, pocket microscopes that cost $10-$20 do a good job of magnifying trichomes so they are large enough to easily tell what color they are.
There are models that do a better job and magnify the trichomes to a larger size, but they tend to be more expensive than $20.
Pocket microscopes that usually sell for under $10 generally don't magnify items as large as more expensive ones. If you have very good eyesight a cheaper microscope might be adequate.
Most have a built-in light, those without require a source of white light. If you are viewing under light that is any color other than near white, you will have a hard time telling what colors the trichomes are.
B) --- Digital Microscope - When hooked up to a computer via the usb port, a digital microscope will display a magnified image on your computer screen. This one has a dial to adjust the focus and provides its own white colored light.
A major benefit of a digital microscope like this is that you can take photos of the trichomes and save them to your computer, to analyze and for reference with future crops.
You can find a good digital microscope for viewing trichomes for about $20. Instructions on using the particular microscope, shown in the image, to view trichomes are here. Links to photographs of trichomes taken with this microscope can be found on the same page.
C) --- Jeweler's Loupe - This method doesn't work for me, can see the trichomes although not large enough to get accurate details, but there are a small group of who say they like a jeweler's loupe.
Avoid loupes and magnifying glasses for viewing trichomes unless you are familiar with, and think you might get accurate results.
This doesn't mean that loupes and magnifying glasses have no function, they are great for monitoring the buds during flowering, but not enough magnification for the tiny trichomes.
There are growers who can determine when to harvest with just a loupe or magnifying glass, but this comes from years of practical experience.
There are cheap loupes, but in most cases with cheap loupes the manufacturer overstates the amount of magnification.
A better quality, usually more expensive, loupe rated at 10x will usually magnify more than a cheap loupe with a rating of 30x or higher.