sAma can be instantly told apart from Arabhi and dEvagAndhAri by the fact that it doesn't use the niSAda. It differs from Shuddha sAvEri vis-a-vis the gAndhAra, as srikant has pointed out.
But let me take this opportunity to narrate the story of sAma, and how like an amoeba engulfing a food particle, it gulped down naTanArAyani, as several sangeetakalAnidhi-s egged it on. Having swallowed natanArAyaNi it coughed up some of its own phrases, and is now an anaemic, washed out srmpds/sdpmgrs.
There used to be a sAma, with interesting vakra-varjya character, replete with rmpmgr, rpmdd, rmpmdd and rmpd phrases in equal measure, that was in vogue. It would occasionally coo a sweet srgs.
This was probably the sAma anammayya sang, and was definitely the sAma used by tyAgayya, dIksitar and kuppayyar. Our ladder-loving mahAvidvAns, with their penchant for straight scales decided that this was all unnecessary and now we have a song of swati tirunAl, a sadAshiva brahmEndra piece and a nandanAr charitra kIrtanam [all retuned in the last century] to represent the current scalariform sAma.
All these have the same srmpds/sdpmgrs(srgs). There is also a varnam by another kalAnidhi that was the last nail in the twin coffins of the old sAma as well as naTanArAyaNI. It firmly established the sr-rm-mp-pds ang of sAma.
mgrs in these rAga-s.
The G in Arabhi is an alpa or fleeting gAndhAra it is always leaning on the M. So a notation of MGRS is actually sung/ played MmgRS. I think this phrase has no place in dEvagAndhAri, though it is used a lot nowadays, as srikant has pointed out.
One example of such contamination is the pallavi of karunAsamudra where many sing Mmgrr,, in a style that almost matches the pallavi of O rAjIvAkSa. The phrase should rather be MG,,rr,,. However there are AlApanas by several stalwarts that are equally Arabhi and dEvagAndhAri!
There is only mrs in suddha sAvEri.
dEvagAndhAri uses a clearly enunciated G, even a dIrgha g - srgsr,,; srg,,rr; srmg,,rr,,etc.
Dr PinAkapANi had mentioned to me that the first phrase in that set, srgsr,, is a sAma phrase, and has no business being in dEvagAndhAri. I feel that singing the first r flat or s dependent can tell apart the sAma and DG versions respectively.
DG has an important pmd,, molecule, that is pronounced differently in sama, when present.
There is a lingering on the m in sAma, and m often occurs in dyads or triads in this rAga.