For preliminary test (if for identification), would  begin with the basic biochemical tests eg. catalase, oxidase, motility. Coupled with information on their morphology (eg. gram staining, shape etc); later you will decide on which further test you will carry out. Further ribosomal rDNA sequencing would complement the biochemical identification.

For thermophilic species, you will also test growth at different temperatures to look at optimum growh temperature range.

thank's for your answer abdul hamid

I think the most important test for thermophilic bacteria is the ability to grow at different temperature. First you must determine what is the suitable range of temperature for growth for your isolates. You must determine if your isolate are thermotolerante or thermophilic or obligate thermophilic. You also test the ability to survive at high temperatures. You can test (if your isolates form endospore) the ability of endospore to survive at high temperature. The classical tests such as cultural properties, morphological properties, Gram stain, endopsore stain, and physiologiacl properties can be determined after determination the suitable growth temeprature. After these tests, you can go ahead for the molecular suitable technique., Good luck

thank you very much for your help

With my experience working on thermophiles, if you are working in Bacillus species then following standard Bergeys manual will be helpful and time efficient. In contrast if you are working on thermus species or geobacillus species then sequencing of 16S rRNA will provide detail information. Also you should try to perform different tests to determine optimum temperature, pH and osmo tolerant of the isolates.