As already stated, this site does not replace a Rabbi. But here are some sources.
The Mishan in Mas. Orla (3:10) says:
ספק העורלה--בארץ ישראל אסור, ובסוריה מותר; ובחוצה לארץ יורד ולוקח, ובלבד שלא יראנו לוקט.
If in doubt, then fruit that could be Orla are forbidden in Eretz Yisrael, but permitted in the Diaspora.
That's also have the Shulchan Aruch paskens in Yoreh Deah 294:9
סְפֵק עָרְלָה בְּאֶרֶץ יִשְׂרָאֵל, אָסוּר. וּבְחוּצָה לָאָרֶץ, מֻתָּר
However, not all fruit in Israel are Safek Orla. There are published lists - for example on Jerusalem Kosher News you can find the following:
Following is a list of summer fruits and those listed as not problematic regarding orla may be purchased from any vendor, even those without a teudat hechsher. The fruits listed as problematic must only be purchased from a vendor with a reliable hechsher regarding Orla.
Those listed as ‘safe’ regarding Orla:
- Chinese oranges (cumquat)
- Grapes (pitted)
- Peach (with the exception of the סמר-סנו variety, which is shaped like a bottle, usually found in July.
- Feijoa (Miniature lemons I believe)
Limequat (cross between a lime and a kumquat)
Fruits requiring a hechsher regarding Orla:
- Grapes (seedless) two varieties are problematic (1) Melissa – long grape, whitish in color, available from July – September (2) Autumn Royal – dark grape which is available from August – September.
- Prickly pear cactus – preferable to buy with a hechsher
- Pitaya – preferable to buy with a hechsher
- Starfruit (or Carambola)
There's also the following useful information on the Star-K site:
How does the Israeli housewife tackle the problem of orla in the marketplace? There are many opinions to approach this issue. The following are the most common:
The most lenient approach is that of the Chazon Ish zt"l who says that since the majority of fruits grown in Israel are not orla, the halacha gives us the right to assume that the fruits sold in the market are from the kosher majority.
The second, more strict, opinion is of the Minchas Shlomo stating that since the question whether or not a fruit is orla originates in the orchard. In the eyes of halacha each fruit is viewed as having a 50% chance of being orla. Without specific knowledge that the fruits are kosher, doubtful fruits would be prohibited.
The strictest opinion is based on the halacha that states that if the total orla volume of fruit in the marketplace is less than 1/2%, the fruit is permitted. This is based on the halacha that if orla is co-mingled with kosher fruit, it is nullified in a 200 to 1 ratio (1/2%). Most mehadrin hechsherim in Eretz Yisroel defer to this opinion.
Please note that this site makes no guarantee of validity, and does not offer professional (particularly rabbinic) advice. Treat information from this site like it came from a crowd of your friends, and use it as background information to take with you when you ask this halachic question of your own rabbi.