I did a lot of reading on detecting counterfeit postage stamps.
I'm referring to current Forever Stamps, not old collectables.
Counterfeiters are getting better all the time.
Real stamps are close to perfection, but if a stamp starts to look questionable, consider where you bought it.
What to look for is:
1) Micro printing showing USPS OR Forever (10x magnifying glass needed)
2) Ultra violet flashlight will show a greenish coating on real stamps / fakes would be bluish.
Put 'real' and fake stamps on a sheet of white paper - you will see the difference.
3) Sharp images (not a dot pattern)
4) Paper feel, texture and gloss
5) Sheets - Look where the four corners meet, should be neat and even
6) Spelling, colors and general appearance
If possible, compare with the same series stamp from Post Office.