nematodes normally cause ROOT KNOT hence uproot you crop showing wilting symptom then look to the roots, if there is knot that is nematodes.

don't mix root knot and nodules for leguminous plant- they are different

Yes, check for presence of root knots on selectively uprooted plants.

During fieldwork, you'll note the plant's yellowing and general weakness, this yellow plant appear in the field with the shape of circular patches. Root-knot or cyst will appear in the case, nematodes infected the root

Nematode always present in patches so, symptoms on crops like yellowing, wilting, stunting will also appear in patches or uneven symptoms in crops. for further confirmation uprooted plants showing galls on roots in case of root-knot nematode and white female will be visible on roots in case of cyst nematodes at a specific time.

BY observing the symptoms on the plants. This will give you a clue if the nematodes are present in the soil or not

I am in agreement with Sajjad. The clue for infected plants by nematodes comes from the weakness of plants. Then we can go through root sampling and check if we can find nematodes in the root glands, of course if it exists.

Ht. Decemson Normally Nematodes symptoms occur in Patches. So, if you notice any patches in the field, this is a clue that there must be a nematode attack so check for soil sampling and root sampling as well.

Find herewith root-knot symptoms in the vegetable field for your guidance.

Hope this will help you.

Thank you.