Those 2 breeds should look different, but with today's bad breeding they often can not be distinguished. Also many mixes look the same, o I find it impossible to tell them apart with certainty.

BUT!

A shih Tzu should be smaller and a Lhasa larger.
A Shih Tzu has a very short nose, possibly even flat, a Lhasa has a bit of a longer nose.
Lhasas almost always have crooked (bowed) front legs.