Ladybird - spiders are easy
Males have boxing gloves, girls don't
And that is actually the true way you tell the difference between them .... (ripped off the web, but describes the differences well):
Sexual dimorphism (differences) is highly variable among spiders. The primary visible differences between male and female spiders are:
females have larger/bulkier bodies and/or the entire female is much larger than the male.
males generally have enlarged palp ends.
Females are usually larger than the male as the physical demands and accompanying energy requirements to produce webs and broods, are far greater. The male is smaller because once mature, his only function is to mate.
Where males are similar in size to the females, they are generally more slender, with longer legs (Selenopidae, Sparassidae, Pachygnatha, Tetragnathidae). In some cases the male can be one thousandth of the weight of the female (Thomisus, Thomisidae; Nephila,Tetragnathidae; Argiope, Araneidae). In other cases, the male and female are so different that they may even appear to be different species where males of certain species mimic wasps or ants (Salticidae; Gnaphosidae; Seothyra, Eresidae).
2. Copulatory organs
The copulatory organs of the Araneomorpha, or true spiders, have entelegyne features.
The male palps are enlarged distally (at the ends) due to a complex copulatory organs or genital bulbs that resemble boxing gloves. Some tiny male spiders have ridiculously large palps relative to their body size. The male and female genital organs are very specific and function on a "lock and key" principle. These organs are used to identify spiders to species level. The female genitalia, the epigyne, is situated ventrally (underside) between the booklung slits on the epigastric furrow. The epigyne is a black, shiny, chitinous, oval to round plate with two openings.
In the primitive spiders, for example Mygalomorpha and haplogyne Araneomorpha, the genitalia have haplogyne features where the female has no epigyne and the males no complex copulatory palps. Instead, the male has a simple bulb that resembles an elongated sting of a scorpion and this is held against the ventral surface of the palp and is not easily visible. Due to the absence of visible genitalia in this group, other characters must be used to determine species.