With time and experience you will see the difference. I really don't know how to explain it to you.
Sempervivum 'Abba' is one you might confuse with a tender succulent such as this Echeveria. But if you see them in person you will see the difference in texture and leaf structure. When they bloom is a real give away that they are different from each other.
Sempervivum bloom and an Echeveria bloom. Sorry the echie bloom is not a very good photo. Maybe some one has a better photo of an echie bloom they can post for Hetty to see.
Here is a better explaination quoted from UBC Botanical
http://www.botanicalgarden.ubc...
Both genera are usually rosulate perennials but some Echeveria species are shrubby.
When you see the two in bloom there is no mistaking them.
Echeveria have flower stems that arise from the sides of the rosettes and have tubular five parted flowers. The rosettes can produce flower stems for years without dying.
Sempervivum have star-like flowers with many more than five petals. The flower stems arise directly from the center of the rosette and the entire plant dies after flowering.
