Polarity of feedback in a feedback circuit
There are three sets of negative feedback in this circuit; one associated with M3 (i.e. the connection from drain to gate), one associated with M2 via R1 and R2 and a subtler one associated with M1 and M2 sharing a common source connection.

How you would recognize them is a matter of experience and learning.

The most basic method is to think about the MOSFETs as having an inverting voltage gain between gate and drain. This then explains M3's local feedback and M2's local feedback.

As for M1's feedback, Vout increases due to Vin increasing and this in turn causes M2 to try and conduct more (due to its local neg feedback via R1 and R2). This then has the effect of lifting both M1 and M2 sources and reducing the drain current of M1 and this of course is effectively a reduction in the real gate-source voltage of M1 and hence is equivalent to Vin reducing. It's is negative feedback.