1) Comparison to the real thing. This is the easiest step any deviation is a red flag. Technically you can use a similar card printed in the same year. Or just get an amazing image of a graded card online
2) Post Image of the card online and ask for help. Most people will gladly help you spot the obvious forgery especially if you get good images.
3) Magnification is a very powerful tool. Learning how Pokemon cards are built is the fool proof way of detecting fake Pokemon cards. Most fakers will not bother with high end details
4) Grade high end cards. Most grading companies have high end tools that you will never bother buying.
5) There are Error Cards And Weirdly Unique Cards. Some cards might look fake but actually a real Pokemon card. This was mine mistake with legendary collection, I thougth they where fake back in the day.