Can I tell if a rash is dangerous by looking at it?

Rashes. Basically the answer to your question is, "no". Making these diagnoses is the parlance of the dermatologist. Parlance of the lay patient is to bring the rash to us for analysis. The same is true with any bump or change in the skin of which you are unsure.
No. There are a large number of skin diseases and if you ar concerned with an unusual rash if it is dangerous it would be prudent ot have it professionally checked before it gets worse or spreads.