How do I perform thin-layer chromatography to identify alkaloid content in fruits?
TLC in alkaloid research

Both analytical and preparative TLC have been used to separate and identify alkaloids from plant material. 

Pretreatment of TLC plates and adsorbents

If you are using silica plates, it is often advantageous to wave the plate above an open bottle with aqueous ammonia or pretreat the plates with an organic solvent containing some triethylamine. Otherwise, the $\mathrm{R_f}$ values of the alkaloids might be too small.

Alkaloid detection on a TLC

As pointed out by Zhe in a comment, this is crucial for preparative TLC. Usually, you run an analytical TLC (same material and mobile phase) in parallel and stain the analytical TLC using Dragendorff's reagent.

General hints on the extraction

Process different parts of a plant separately

Different parts of a plants have different functions. The distribution of particular compounds or compounds classes, such as


chlorophylls and carotenoids
quinones
polyphenols
carbohydrates
terpenoids
alkaloids


will vary in flowers, fruits, seeds, leaves, stems (and bark), roots, and tubers. 

It is therefore reasonable to process the different parts of a plant separately.

Use dry plant material

The water that you remove in the beginning doesn't disturb the extraction in a later stage. Dry the plant material. If you assume that the compounds you are looking for are air-sensitive, dry the material in vacuum.

Use powdered plant material

Extraction is more efficient on solid materials with a larger surface.

Use the right solvent

Two different strategies are conceivable:


Extract all soluble compounds with a polar protic solvent (methanol, ethanol) at once and partition between solvents of different polarity later
Leach the dry plant material stepwise with solvents of increasing polarity:


cyclohexane
ethyl acetate
chloroform
methanol



Use the right technique

You can 


stir/shake the dried plant material with a solvent at room temperature
boil plant the material in a solvent
use a Soxhlet extractor


If necessary, perform the extraction under a nitrogen or argon atmosphere

Specific advice on extraction

Rambutan (Nephelium lappaceum)

In Antioxidants, 2014, 3, 371-386 (Open Access article), the authors write


  The fruit samples were washed and air-dried, followed by drying in an oven at 40 °C for 24 h. All the dried samples were ground to fine powder with a grinder, after which they were vacuum packed into a nylon-linear low density polyethylene film before analysis. An extraction with 80%
  ethanol was carried out, and the mixture was shaken at 50 °C for 120 min using an orbital shaking incubator as optimized in a previous study. The residue was subjected to another round of extraction under similar conditions to obtain maximal yield of antioxidant activities. The extracts were combined and centrifuged, followed by filtration and concentration of the supernatant using a rotary evaporator at 45 °C. The concentrated extract was freeze-dried, wrapped with aluminum foil and kept at −20 °C. The crude extract as then partially fractionated using silica packed open column chromatography and using increasing polarity of solvent, ethyl acetate, chloroform and methanol (100:0:0 to 0:0:100 v/v).


In the article cited above, alkaloids were not mentioned.

Ambarella (Spondias dulcis)

In Asian Pac J Trop Biomed., 2013, 3, 682-691, the authors give the following procedure


  The pulverized plant materials were soaked in 3.5 L of methanol in separate containers and kept for 1 week with occasional shaking. The whole mixtures were then filtered through sterilized cotton followed by Whatman No. 1 filter paper and concentrated using a rotary evaporator (IKA , Germany). The weight of crude extracts of fruits and leaves were 135 g and 101 g, respectively. A total of 40 g crude extract of each plant materials was then partitioned with chloroform and dichloromethane. The percentage
  yields of fruit and leaves were methanol ( 21.3 % , 15.8 %), chloroform ( 19.5 % , 23.1 %) and dichloromethane (20.9 % , 25.4 %).


The authors mention that extracts both from the fruits and the leaves of S. sulcis gave a positive test with Dragendorff's reagent, but do not provide further information on the amount and/or nature of the alkaloids.