Its assur to separate Terumah on shabbos with a Maaseh. But is it also Assur to be "Torem" Terumah , just by looking and by Machshava (Which works on a weekday) ? Or is that considered the melacha of Makeh BPatish ? where in the gemara can I find this sugya ?

ב"ה Shalom, The Sugya of the "issur" of taking Terumot and ma'asrot on Shabbat is found in Beitza 36bביצה דף לו: and is brought in the Rambam שבת כג:יד , and in Shulchan Aruch הלכות שבת שלט:ד . One of the reasons forbidding this brought in the Rambam is מתקן מנא which is as if he has fixed a utensil, which until now was unfit for use. The Sugya of Torem by Machshava is in Gittin גיטין לא. , see long discussions in Rishonim and Achronim , what can be done by machshava. Is it only Teruma gedola? Does it include Termat Maaser, Ma'asrot ec. The issue of מתקן מנא is connected to Makeh BPatish and that itself is another discussion spread in different sugyot , one in Beitza דף לד. 34. In Shemirat Shabbat Kehilchata (Intro. 2:2) the taking of Terumot and masrot on Shabbat is forbidden דרבנן by Rabbinical decree, and in in Piskei Teshuvot הלכות שבת שלט:ט it includes taking of Terumot and ma'asrot bmachshava. All the Best