varietyvista.com
doubleddie.com
coppercoins.com
Look at the ERTY in LIBERTY and the 20 in the date for minor DDOs.
Look at the Lincoln Statue and the two columns next to it for minor DDRs on memorial cents.
Look around the middle stripe on the shield and at the designers initials for minor DDRs on shield cents.
Some early 2000s DDRs have doubling or extra thickness on E PLURIBUS UNUM and or the bottoms of the column bays.
There is a strong 2006 doubled ear and a few DDOs from 2002-2006 have some of the beard details doubled.
A few DDRs have doubling around some of the outer columns and one 2005
DDR has the design above the 5th column doubled.
I think the best ones to look for are:
2000
Wide AM2004
DDR - Extra thickness on E PLURIBUS UNUM
2006
DDO - Doubled Ear
2006
DDO - All lettering
2011
DDO - All lettering (Multiple strong ones)
2014
DDO - All lettering (Multiple, FS-101 is rare)
2015
DDO - All lettering (Multiple)
These are just the few of the big ones that you'll hear of and you'll easily spot.