Reference for correct usage
I would have to guess that your plant was dying because you have been over watering,
This is incorrect grammar if the intention is to use the 3rd conditional with the verb infinitive "to guess". The correct grammar is to say "I would have guessed". However that does not work with this sentence because it is not meant to be in the past, the sentence is meant for the present. The conditional present however would work and "I would guess that your plant..." is correct. This implies that they are not sure but "would guess" something.
Reference for actual sentence
To be obliged to; must:
The sentence however reads okay, reading it I would assume that the person commenting is referring to a "hypothetical state of affairs" in the present tense. So they may have meant to use the conditional for the infinitive "to have". Whilst "have" is "used with a past participle to form the perfect, pluperfect, and future perfect tenses, and the conditional mood", it is also a verb in its own right.
Per the reference above, the 14th definition of have means "to be obliged to". So the comment probably means:
If I was obliged to guess then...
If I was forced to guess then...
So using the conditional present of the verb "to have" together with the infinitive "to guess" works.