We are told:
"Regulation of the sacred liturgy depends solely on the authority of the Church, that is, on the Apostolic See and, as laws may determine, on the bishop. Therefore no other person, even if he be a priest, may add, remove, or change anything in the liturgy on his own authority” (Constitution on the Sacred Liturgy, 22).
This means do not monkey around with how to worship God (people go to eternal Hell for that kind of thing, just saying...).
Here are a few keys signals that liturgical abuses are happening:
1) The priest is ad-libbing (anything other than his homily, and even that should have some notes)
2) He randomly and intentionally skips certain portions of the Mass (make sure you know which parts are supposed to be skipped in certain days of the Calendar)
3) He refuses to serve communion on the tongue
4) He interrupts the Mass to add in his explanation of things or comments
5) He allows laity to participate in the consecration
6) He gives his personal opinion as a "homily" instead of teaching the dogmas of the Church
7) He draws attention to himself during the Mass like an entertainer
8) He allows liturgical dance in the Mass
9) He never allows Latin or Gregorian Chant in the Mass
10) He comes up with new and fancy innovations to make Mass interesting and enjoyable
Some of these are more serious than others, but they are all grave abuses of the worship of Almighty God. If any one of these occurs it is reason for suspicion. If more than one, or just one repeatedly, occurs, then it is probably best to ask for God to be merciful on their souls and find another place to attend Mass.